The Mets entered Thursday with a bullpen ERA of 5.37 ERA. Only the Orioles and Nationals had a worse mark.

With Seth Lugo and Edwin Diaz perhaps unavailable after pitching in Wednesday’s loss to the Reds and Jeurys Familia on the injured list with a shoulder issue, Mickey Callaway admitted he didn’t exactly have a plan for how to finish the game once Noah Syndergaard’s day was over.

“We have to count on other guys on occasion,’’ Callaway said before the game. “This might be one of those days.’’

Fortunately for Callaway and the Mets, Syndergaard finished what he started in a 1-0, complete-game victory, and the manager avoided having to make a tough decision.

“I don’t know how we’d get through the rest of the game if he doesn’t complete it,’’ Callaway said with a slight chuckle after Syndergaard’s 10-strikeout, four-hit masterpiece at Citi Field.

“The lack of relief was a relief today,’’ the manager said. “I’m not sure what we were going to do.’’

Diaz, in addition to having tossed an inning Wednesday, had allowed a go-ahead homer in each of his past two appearances.

Callaway said he would have had to rely on Robert Gsellman, who has struggled in two of his last three outings, or Luis Avilan, who hasn’t pitched since Friday.

“We would have had to do something out of the ordinary today,’’ Callaway said. “Somebody was going to have to do something special, and Noah stepped up and got it done.’’

Callaway’s bullpen reality is why he not only let Syndergaard hit in the bottom of the eighth — with a one-run lead he provided with an opposite-field homer to left to lead off the third — but why he was going to stick with the right-hander even if he allowed more than one base runner in the ninth.

“If things had started getting hairy, I would have given him a chance to get out of it,’’ Callaway said. “He deserved it at that point. Maybe once they tied it up and he was getting to around 112-115 [pitches], I would have had to go ahead and get Gsellman in there, but when you go that deep and work that hard, you deserve to finish it, or to let him at least attempt that.’’

That’s just what Syndergaard did.

He gave up a two-out single to Derek Dietrich, and after Michael Lorenzen pinch ran for Dietrich and stole second for the Reds’ first runner of the day in scoring position, Syndergaard got Yasiel Puig looking to end the game.