Frankie Montas was smacked around in six rehab starts at High-A Brooklyn and Triple-A Syracuse: eight home runs allowed, a 12.05 ERA and a 2.14 WHIP in 18 2/3 innings. He did not appear major league-ready, but the nine-year veteran knew better.

“I’ve said it before,” Montas said Tuesday after tossing five scoreless innings in his Mets debut against the Braves. “When I was in the minors, I was just trying to get my pitches in, get my pitch count, work on my pitches and trying to get ready for when I get opportunity up here and I was just trying to go out there and do my job, get people out.”

Montas struck out five, walked three, gave up three hits and threw 46 of 80 pitches for strikes in front of 38,130 at Citi Field. He got nine whiffs on his four-seam fastball, the second most in his career, and left with a 3-0 lead.

“I thought he was good,” manager Carlos Mendoza told reporters. “I thought the ball was coming out well. A heavy sinker, 97, 98, attacked. I thought there was some cutters, some sliders. They didn’t chase his split, but overall, got groundballs there. Threw strikes. He looked strong so I thought he gave us a chance there.”

The bullpen faltered, however, coughing up five runs in the sixth inning, and spoiling a chance for Montas’ first win as a Met. The Braves went on to a 7-4 victory. New York (46-34) has lost five times to Atlanta in eight days, 10 of 11 games, and dropped to 1 1/2 games behind the Phillies in the division.

“Every team goes through that,” Montas said. “Every team at some point in the year they go through those stretches that like it kinda look like guys can’t figure out. But at the end of the day, I rather that happen now than later on in the season or in the playoffs.”

Montas, 32, signed a two-year, $34 million deal with the Mets in December with a player option after this season after going 7-11 with a 4.84 ERA with the Reds and Brewers last year. He impressed Mendoza in the Wild Card round when he threw 3 2/3 innings and gave up one earned run against the Mets.

“That’s kind of like the guy that we saw in the playoffs … with the way that the ball was coming out. It’s easy 97, it’s a heavy sinker, he’s around the zone. He’s got the cutter-slider and obviously we know the split, which I thought today, they didn’t bite, but if we get a guy like that he’s gonna help us, that’s why he’s here.”