With his team swooning, and questions swirling about his own recent performance, Frankie Montas made his Mets debut and, fittingly for the hottest game day in Citi Field history, tried to play firefighter.

Hours after the Mets announced that Sean Manaea had suffered a minor setback in his own return from injury, Montas’ outing against Atlanta provided a soothing balm Tuesday. He allowed no runs and three hits with three walks and five strikeouts over five innings; Montas, who was on a pitch count, threw 80 pitches, 46 for strikes. He was set to earn the win before the Mets’ bullpen imploded in the sixth.

His performance was somewhat unexpected, at least considering what he’d done up to this point.

Montas, who signed this offseason for two years, $34 million but suffered a lat injury in spring training, was out in Triple-A Syracuse making minor leaguers look like batting champs during his rehab starts (four in Syracuse, two at Single-A Brooklyn): In six games, he had a 12.05 ERA, allowing 25 runs, eight homers and 10 walks, with 12 strikeouts in 18 2⁄3 innings.

Eventually, manager Carlos Mendoza allowed that when Montas returned, it would potentially be to the bullpen. Last week, he even openly wondered if Montas would be activated when his rehab clock expired, or if the poor showing indicated a lingering injury. In the days leading up to Montas’ season debut, though, the tune changed.

The injury-ravaged Mets were out of options, and Montas actually had decent stuff in his final rehab outing, president of baseball operations David Stearns said. Mendoza, meanwhile, said he was heartened by Montas’ subsequent bullpen sessions.

“In the final rehab start, stuff- wise, [he] was actually pretty good,” Stearns said Tuesday. “He probably got a little fatigued as he got to the fifth, but we saw the stuff package for the first three to four innings of that game that we wanted to see.”

Montas danced around a walk and a single in both the first and third innings Tuesday but showed positive signs throughout. His fastball, which averaged 95.6 mph last year, topped out at 97.7, throwing 18 of those, nine for swings and misses.

The problem during his rehab was “was just more execution — the way that some of his pitches were moving or acting, whether it was the sweeper, the cutter, the split,” Mendoza said. Montas and pitching coach Jeremy Hefner spent time “creating some game situations as he was throwing his bullpens. We saw better results as far as the characteristics of the pitches the last time out in Triple-A.”