KANSAS CITY, Mo. — Kodai Senga hit the injured list on June 12. It’s a notable date, not only because the Mets lost one of the strongest weapons in their arsenal but because it marked the beginning of a worrisome decline.

They had a season-high 5 1⁄2-game lead in the NL East on that day, and on the next, they kicked off a seven-game losing streak that would sink their June. Between then and now, the following pitchers hit the injured list: Tylor Megill, Dedniel Nunez, Max Kranick, Griffin Canning, Paul Blackburn and Jose Butto. Canning is out for the season. Nunez might be, too.

Oh, and an MRI discovered loose bodies in Sean Manaea’s elbow, which slowed his return from an oblique injury.

So there was a certain cleaning quality to Senga’s return to the mound at Kauffman Stadium on Friday night. The Mets hardly are cured — they did drop both games of a doubleheader to the last-place Orioles on Thursday — but despite their travails, they entered the day 1½ games behind the Phillies for first place.

Senga, who Carlos Mendoza said before the game would be on a pitch count, worked through heavy traffic in his first three innings but didn’t allow a run before retiring the side in order in his fourth and final inning. He allowed four hits with two walks and four strikeouts and threw 67 pitches, 42 for strikes.

Although his velocity was down nearly across the board, he did get swings and misses on his ghost fork. All of his strikeouts were swinging strike threes, and three of them were forkballs.

Manaea, who on Friday said he felt good after receiving a cortisone shot in his left elbow a little less than three weeks ago, is slated to make his season debut on Sunday, pitching behind Clay Holmes, who will start.

“I hate being on the IL,” he said. “I want to be out there helping this team win, and when you can’t do that, it’s really frustrating .  .  . It feels good to finally be back.”

Between the two, the Mets have a righthander in Senga whose 1.47 ERA going into Friday was the lowest of any starter with a minimum of 60 innings pitched and a lefthander in Manaea who emerged as the de facto ace during last year’s postseason push.

Add to that: Frankie Montas showed improvement in his latest start against the Yankees, Holmes has stayed durable despite concerns about his transition to a starting role and David Peterson was named to his first All-Star Game.

All of which undoubtedly is good news for a pitching staff that was among the best, if not the best, in baseball for two months but has pitched to a 5.68 ERA in the past 30 days — third worst behind the Rockies and the Nationals.

“It’s a big boost, getting Senga back, getting Sean back,” Mendoza said. “I’ve been saying it. They’re a big part of this team, and for us to be able to get those guys before the break, obviously, it’s important to us. We’re going to need them.”

And if there’s some sort of bright side to injury, it’s that Senga and Manaea will be fresh for the second half. Senga was hurt last year, but Manaea, who logged a career-high 181 2⁄3 innings in 2024, was visibly fatigued when the playoffs finally rolled around.

Time will tell how he deals with the lingering loose bodies in his elbow, but “we’ve been staying on top of it, doing all the stuff the training staff wants me to do,” he said. “Our boys have been grinding, so to be able to be fresh for the second half is a positive.”

His teammates agree.

“It’s awesome,” Brandon Nimmo said. “We’ve been looking for Sean this whole year — want him to stay healthy and would love for him to do some of the stuff that he did last year. [That would] be amazing. So obviously, it would be a real help.

“And then Senga, the same thing. When he’s out there, he gives us a chance to win every time. So you definitely like your chances with those guys going out there.”

Game 1 of Thursday’s doubleheader marked Nimmo’s 1,000th career game. Only four players in Mets history have played 1,000 games, hit at least 100 homers and walked at least 500 times: Nimmo, David Wright, Darryl Strawberry and Howard Johnson.

Nimmo realized Wednesday night that he was about to hit the milestone “and it gave me time to reflect on it,” he said. “I see the pictures from when I debuted to now, and [seeing] just how much I’ve grown up in the organization . . . I actually became a man in this organization, so it means a lot to me. I hope this fan base has enjoyed the ride as much as I have, and there’s more to come — hopefully a lot more good things to come.”

Jesse Winker was placed on the 10-day IL with back inflammation. He’ll treat the area with medication for four to seven days before the Mets evaluate him again. Mendoza said the injury is unrelated to the oblique injury that sidelined him for more than two months .  .  . The MRI on Starling Marte’s injured knee was “very encouraging,” Mendoza said. He’s set to return after the All-Star break . . .  Justin Hagenman was optioned to Triple-A Syracuse . . .  DH Jared Young was recalled . . .  Reliever Rico Garcia was designated for assignment.