KANSAS CITY, Mo. — A day that started with so much promise was beginning to sour.

After nearly a month on the injured list, Kodai Senga made his long-awaited return Friday night — two days before Sean Manaea is set to do the same — but as the innings went on, a theme emerged. You can have all the pitching you want, but you do, in fact, have to score runs to win a baseball game.

It hasn’t always been a strong suit for the Mets, especially with runners in scoring position, but one swing by Mark Vientos changed the narrative, salvaged the evening and lifted the Mets to an 8-3 comeback win over the Royals at Kauffman Stadium on Friday night.

The Mets were trailing 3-1 in the eighth when Royals reliever Steven Cruz loaded the bases with none out, allowing a single by Brandon Nimmo and back-to-back walks to Francisco Lindor and Juan Soto. But for a team that came into the day hitting .227 with runners in scoring position — third-worst in baseball — there are no guarantees. So when closer Carlos Estevez struck out Pete Alonso swinging to bring up Vientos, it felt as if the script was set.

Instead, Vientos, who was batting .119 in his previous 13 games, muscled Estevez’s 2-and-2 slider off the end of the bat the other way for a three-run double that gave the Mets a 4-3 lead.

“It was good to redeem myself” after striking out with the bases loaded in the third, Vientos said. “I’ve got a good group of people around me pushing for me, keeping me motivated, and they have confidence in me, so it feels good to have a team [have] my back.”

The Mets then went back-to-back in the ninth: Francisco Lindor tacked on a three-run homer off Taylor Clarke (No. 19) and Soto followed with his team-leading 22nd. It was the sixth time this season the Mets have hit back-to-back homers.

So ended the roller coaster that started when Senga, sidelined since June 12 with a hamstring strain, pitched four scoreless but very eventful innings. It continued through a series of misplays and poor at-bats and seemed to crash when Huascar Brazoban allowed a go-ahead two-run homer by Bobby Witt Jr. in the seventh.

The Mets loaded the bases with one out against Michael Wacha in the third, and though he struck out Soto swinging, he wasn’t quite so lucky against Alonso, who walked to force home the game’s opening run after an eight-pitch at-bat. But in what’s been an ongoing theme, the Mets could do no more with runners in scoring position as Vientos struck out on three pitches.

“For Mark to get that big one [later in the game], it was huge for us. It was huge for him, too,” Carlos Mendoza said. “That’s part of being a big-league player. You have to go through the ups and downs. You have to find a way to keep fighting.”

Senga, who was on a pitch count, worked through heavy traffic in his first three innings but didn’t allow a run and retired the side in order in the fourth. He allowed four hits with two walks and four strikeouts and threw 67 pitches, 42 for strikes.

Although his velocity was down slightly, he did get swings and misses on his ghost fork. All of his strikeouts were swinging strike threes, and three of those four pitches were forkballs.

“I was a little bit nervous getting back out on the mound,” Senga said through an interpreter. “There’s some things to reflect on. I could have done better, but the biggest thing is that I was out there. I’m back here and I’m healthy.”

The Royals tied the score off reliever Alex Carrillo in the sixth thanks to a misplay by Nimmo in left. Perez stroked a one-out single to left and, with two outs, Nick Loftin hit a drive on which Nimmo went into a slide and was unable to backhand. Thanks to the risky attempt and Kauffman’s cavernous outfield, the 35-year-old catcher scored easily.

Then, with one out and one on in the seventh, Witt blasted Brazoban’s grooved 96.6-mph fastball 431 feet to centerfield to put the Royals up 3-1.

Vientos, however, ensured that the Mets still can close out the first half in positive fashion.

He’s “a good hitter,” Mendoza said. “It’s taking a little bit longer than he wanted . . . [but] we’re going to need him.”

Clay Holmes will start Sunday and Manaea will piggyback off of him, Mendoza said . . . 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.