CINCINNATI — Kodai Senga really needed a mulligan on his first inning back with the Mets, but it turns out do-overs aren’t allowed.

The beleaguered right-hander had an ugly opening frame Tuesday in his return to a major league mound and followed with three improved innings, but by then it was too late.

Senga’s clunker, coupled with a mediocre offensive showing, sank the Mets in their 5-3 loss to the Reds. The Mets lost their second straight and now just hope to avoid a series sweep.

Thrust into the rotation after Christian Scott was placed on the injured list with a right hip impingement, Senga allowed two homers for four runs in the first inning. Overall, he allowed four runs on two hits and four walks with five strikeouts over four innings.

It was the kind of performance the Mets have received all too often from Senga, who was returning from a back injury that kept him on the injured list for six weeks.

Manager Carlos Mendoza was ejected in the seventh inning for arguing with plate umpire Carlos Torres’ strike zone. The Mets were out of ABS challenges after losing both by the second inning.

Senga walked Blake Dunn and JJ Bleday in succession to begin his night before Sal Stewart blasted a 2-1 sinker over the left field fence for a three-run homer. Senga got two outs in the inning — one on A.J. Ewing’s diving catch that robbed Nathaniel Lowe — before Spencer Steer homered to bury the Mets in a 4-0 hole.

Bo Bichette’s RBI single in the third sliced the Reds’ lead to 4-1. Brett Baty was hit by a pitch leading off and Francisco Alvarez singled with one out before the sizzling Bichette delivered for his 41st RBI this season.

Bichette began the day on a 10-for-21 (.476) tear. But after a walk to Juan Soto that loaded the bases, Jared Young was retired to kill the Mets’ rally.

The Mets loaded the bases in the fourth with two outs — Ewing and Alvarez both walked with another Baty plunking sandwiched in between — before Carson Benge was retired.

In the fifth, Bichette delivered a leadoff single but Young grounded into an inning-ending double play after Soto struck out.

Stewart’s RBI single in the fifth against Cionel Pérez widened the Mets deficit to 5-1. Edwin Arroyo doubled leading off and Dunn singled before Stewart’s hit.

Mark Vientos smashed a pinch-hit two-run homer in the sixth that pulled the Mets to within 5-3.

Marcus Semien singled in the inning and reached second on Steer’s error in right field before Vientos went deep with two outs.

The Mets had their best chance to tie in the eighth, following walks to Semien and Baty to begin the inning, but Vientos and Alvarez struck out before Benge was retired.