After the trades of Max Scherzer and Justin Verlander last summer, Kodai Senga assumed the role of the Mets’ staff ace, and figured to be the team’s Opening Day starter this year. Already, however, the Mets are in the position of having to adjust. A day after Senga missed a team workout due to what was initially described as arm fatigue, president of baseball operations David Stearns revealed that the 31-year-old righty will start the year on the injured list with a shoulder strain. For as tantalizing as the possibility of a free agent addition may be, the team plans to stay in-house to absorb his absence.

In Wednesday’s media session, manager Carlos Mendoza said that after Tuesday’s side session, Senga told Mets trainers he was experiencing arm fatigue, which is hardly uncommon at this time of year as pitchers build up their workloads. This wasn’t the first time this spring that he had reported fatigue, however, and so the Mets sent him for an MRI, which revealed a moderate posterior capsule strain. He’ll be shut down from throwing, but this isn’t an injury that suggests he’ll need surgery. Even so, Stearns would not offer a timeline for his return. “What I can say at this point, comfortably, is we don’t expect Opening Day,” he told reporters on Thursday. “But I do expect him to make a bunch of starts for us this year… Hopefully we caught it early enough that this is just a speedbump.”

While not presented as a worst-case scenario, that’s still pretty vague as far as what Senga might contribute in 2024, and when. If there’s good news, it’s that this isn’t the type of shoulder injury that Kyle Wright and Brandon Woodruff suffered. As Under the Knife’s Will Carroll pointed out, they had anterior capsule tears that required surgical repair, costing them most of 2023 and likely all of this season. Nonetheless, however long Senga is out, the Mets will have a tough time replacing him.

After 11 seasons with the Fukuoka Softbank Hawks of the Japan Pacific League, Senga joined the Mets on a five-year, $75 million contract. Given his history of pitching only every sixth or seventh day in Japan, the team did its best to provide him a bit of extra rest between starts; only three times did he take a turn on four days of rest, with 17 starts coming on five days, six on six days, and three on more than that. All told, he acquitted himself in impressive fashion, posting a 2.98 ERA and 3.63 FIP in 166.1 innings, making the NL All-Star team, and placing second in the Rookie of the Year voting behind Corbin Carroll and seventh in the Cy Young voting. That ERA ranked second in the league behind Blake Snell’s 2.20, while his 202 strikeouts placed eighth and his 3.4 WAR tied for ninth. Among Japanese-born pitchers who have come stateside, only Hideo Nomo and Yu Darvish had stronger debuts in terms of WAR or strikeouts, with the former totaling 5.2 WAR and 236 K’s for the Dodgers in 1995, while the latter had 4.7 WAR and 221 K’s for the Rangers in 2012.

When Senga joined the Mets, he figured to be the third starter on a contending team behind a pair of three-time Cy Young award winners, but that changed when they traded Scherzer and Verlander. With the Mets’ decision to rein in their payroll — still the majors’ largest in terms of annual salaries ($315 million) and Competitive Balance Tax ($328 million) — this winter, he now fronts a rotation with a much lower ceiling on a team projected to finish around .500. Currently ranked 23rd in projected starting pitcher WAR by our Depth Charts, this group is full of pitchers aspiring to stay healthy, available, and productive for a full season.

The most experienced of the group is 35-year-old lefty José Quintana, who after making at least 31 starts annually from 2013–19 has done so just once in the past three seasons. Last year, his first of a two-year, $26 million deal with the Mets, he was limited to 13 starts by a stress fracture in his rib, one that was revealed to be caused by a benign lesion that required bone graft surgery. He was solid upon his return just after the All-Star break, posting a 3.57 ERA and 3.52 FIP in 75.2 innings. He did a great job of avoiding hard contact, and his sinker was particularly effective, holding hitters to a .198 AVG and .253 SLG in 105 PA. Even if he’s just a five-and-fly guy — he’s averaged less than 5 1/3 innings in each of his last three full seasons (2018, ’19, ’22) — he should provide some welcome stability for the rotation while fulfilling the role of the sage veteran.

The most accomplished of the group is 30-year-old righty Luis Severino, a two-time All-Star who has finished as high as third in the Cy Young voting. But due to shoulder inflammation (2019), Tommy John surgery (2020-21), and a pair of lat strains (2022 and ’23), he’s managed just 40 starts and 209.1 innings over the past five seasons. When he was available last year between a season-opening lat strain and a season-ending oblique strain, he generally struggled, posting a 6.65 ERA and 6.14 FIP in 89.1 innings while serving up a gruesome 2.32 home runs per nine. His four-seam fastball still averaged 96.5 mph, but hitters slugged .680 against it, a problem that may be attributable to his tipping pitches with men on base. Even so, he did have the occasional start that offered hope he could find his way out of such a mess. The Mets signed him to a one-year, $13 million deal on the belief that it’s possible, particularly if he can avoid tipping.

Also tantalizing in terms of velocity is 32-year-old lefty Sean Manaea, who split last season between the bullpen (27 appearances, many of them following an opener) and rotation (10 starts) for the Giants, putting up a 4.40 ERA and 3.90 FIP in 117.2 innings. After spending time at Driveline after the 2022 season, Manaea increased his average four-seam velocity from 91.3 mph to 93.6, and was able to maintain that gain even during his longer outings. He also added a sweeper that held hitters to a .158 AVG and .184 SLG in 42 PA while generating a 36.3% whiff rate. This year, after more work at Driveline, he’s planning to introduce an improved changeup. His existing one held hitters to a .208 AV and .333 SLG in 2023, but generated just an 18.7% whiff rate; with an adjusted grip, he’s hoping to get batters to chase more. He’s also adding a new cutter to serve as a weapon against righties, who hit for a .333 wOBA against him in 2023, compared to the .256 wOBA he allowed to lefties.

The other newcomer from outside the organization is 31-year-old righty Adrian Houser. Acquired from the Brewers on December 20 along with outfielder Tyrone Taylor in a trade that sent righty prospect Coleman Crow to Milwaukee, Houser posted a 4.12 ERA and 3.99 FIP in 111.1 innings last year, making 21 starts and two relief appearances. He didn’t make his season debut until May 7 due to a groin strain, and missed a couple of weeks in late August and early September with elbow inflammation. Houser throws a heavy sinker, generates lots of groundballs, and does a decent job of keeping the ball in the park, though last year’s 1.05 HR/9 was his highest full-season rate since 2019.

Stearns said the battle to replace Senga features Tylor Megill, Joey Lucchesi, and José Butto, all of whom have at least one minor league option remaining. By now the 28-year-old Megill is a familiar face, as he’s pitched for the Mets on and off since debuting in June 2021. In fact, he was pressed into duty as the Opening Day starter in 2022, and three weeks later, he threw the first five innings of a combined no-hitter. His 25 starts last year ranked second on the team, but unfortunately, he was erratic to the point of getting sent down to Triple-A Syracuse for about six weeks. He finished with a 4.70 ERA and 4.96 FIP as he struggled to throw strikes and avoid hard contact. He walked 10.2% of hitters while striking out just 18.5%; between that and the 9% of his batted balls that were barreled, he ended up with a 5.89 xERA. He can bring it with a mid-90s fastball and great extension thanks to his 6-foot-7 frame, but maintaining his velocity has been a challenge. In his final outing of last season, he broke out a split-fingered fastball that he learned from Senga, and he spent the winter working to hone it; he calls it “The American Spork,” referencing Senga’s “Ghost Fork.” “Got a lot of reps with it and it’s working well,” he told reporters this week. “It’s definitely part of the arsenal now.”

Lucchesi, a 30-year-old lefty, has spent parts of five seasons in the majors. He made nine starts totaling 46.2 innings for the Mets last year, turning in a tidy 2.89 ERA that was hardly supported by his peripherals; he struck out just 16.4% of hitters and served up a 10.4% barrel rate en route to a 4.22 FIP and a 5.48 xERA. Butto, a 25-year-old righty, is the least experienced of the three, still a rookie actually. He throws a 93-95 mph fastball with a plus changeup and a good slider; the secondaries both miss bats at an above-average rate. Used as a spot starter last year, he made seven starts and two relief appearances totaling 42 innings, turning in a 3.64 ERA and 4.02 FIP, walking a gaudy 12.8% of hitters (against a modest 21.2% strikeout rate) but holding them to a 2.5% barrel rate and 0.64 HR/9.

Unlikely to figure into the Mets’ season-opening plans but perhaps in play later this year are righties Christian Scott, Mike Vasil, and Dominic Hamel, all of whom Eric Longenhagen covered in the team’s Imminent Big Leaguers piece earlier this month (along with Butto). None of those three are on the 40-man roster; Scott and Hamel are entering their age-25 seasons but haven’t pitched above Double-A, while Vasil is entering his age-24 season and took 16 turns at Syracuse last year, as well as 10 at Binghamton. Via Longenhagen, Scott grades out as the best of them, a 50 FV prospect who projects as a mid-rotation starter thanks to his combination of a 94-95 mph fastball, a plus slider, and a splitter. He’s 6-foot-4, weighs 215 pounds and gets great extension. Hamel, a 45 FV prospect, throws a 92-96 mph fastball and two good breaking balls but has iffy command. Vasil, a 45 FV prospect, projects as “a rock steady no. 4/5 starter on a good team,” per Longehagen; he throws a 92-95 mph fastball, a plus slider, an average curveball and a changeup.

While the free agent market still has no shortage of pitchers who could help the Mets, from expensive options, such as Snell and Jordan Montgomery, to more affordable ones, such as righties Mike Clevinger, Michael Lorenzen, and Zack Greinke, the team’s tax situation makes a signing highly unlikely. Given that they’re already above the fourth-tier threshold of $297 million, the Mets will pay a 110% tax on the salaries of anybody they add. “We’re asking people to step up,” said Stearns, speaking of his internal options.

If enough of those pitchers do step up, the Mets could have an interesting summer, because even with Senga down, they currently project to have a 30.6% chance of making the playoffs. But those odds still depend upon him making a substantial contribution to the team, and right now, that’s anything but guaranteed.

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