CINCINNATI — Todd Frazier is among the Mets’ elder statesmen, but that didn’t stop him from making a rookie mistake Saturday.

The Mets third baseman believed the grounder Aristides Aquino hit to him near the bag in the first inning was foul and delayed in his throw to first base. Aquino was safe for an RBI single that gave the Reds their first run in a 3-2 victory.

“I assumed, and you are not supposed to assume in baseball,” Frazier said. “I made a boneheaded mistake.”

Only exacerbating matters, Frazier had booted Eugenio Suarez’s grounder for an error in the previous at-bat, allowing the Reds to load the bases.

Edwin Diaz could be back in the mix for high-leverage relief appearances, but in small doses.

A day after the beleaguered Mets righty passed a test of sorts by striking out the only batter he faced, manager Mickey Callaway said Saturday he could envision using Diaz to get an important out if needed.

Diaz had entered in the eighth inning and struck out the Suarez with two runners on base in a game the Mets led by five runs.

“I think he’s an option,” Callaway said. “When you look to our bullpen if you need a big strikeout, he’s a guy that is going to come in and get a big strikeout.”

Callaway noted the fact lefties were hitting just .198 against Diaz. But the right-hander has also surrendered five homers to lefties.

“We are aware of the homers,” Callaway said. “That is the chance you will take, if you are looking for the strikeout, need the strikeout to win. But I think we have faith he might be one of our best options if we need in a dire situation to get a big strikeout.”

The manager said multiple factors were involved in his decision to flip Steven Matz and Marcus Stroman in the rotation. Stroman will pitch Sunday at Great American Ball Park and Matz the following day against the Marlins at Citi Field.

“We really thought the way Stroman has thrown in this ballpark versus who they have that can do damage, we felt the matchup was probably the better of the two,” Callaway said. “Get Matzy throwing at home where he’s had so much success and help that psychology play out and Stroman here, get some ground balls and keep the ball down, pitch very similar to Colorado.”

Stroman pitched seven shutout innings in his previous start Tuesday against the Rockies. The lefty Matz has pitched to a 6.62 ERA on the road this season.