ST. LOUIS — This was not the debut the Yankees were looking for from Frankie Montas. 

In his first start since being acquired from Oakland at the trade deadline last week, the right-hander allowed a season-high six runs in just three innings in a 12-9 loss to the Cardinals at Busch Stadium. 

“I wanted to go out there and show them what I can do,’’ Montas said. “That was not the case today. But this is not my last one. This is the first one.” 

With the Yankees looking to snap a four-game losing streak, their longest of the year, Montas gave up a run in the first inning. Then, after the Yankees scored three in the top of the second, the right-hander allowed five runs in the bottom of the inning, highlighted by a three-run home run from Nolan Arenado. 

Montas was on a pitch count, since it was just his third start since being forced from a start on July 1 with right shoulder inflammation. He also hadn’t pitched since July 26 after he was placed on the bereavement list following the death of his wife’s mother. 

He didn’t get to St. Louis from his home in Arizona until Saturday night and was introducing himself to some new teammates Sunday morning. 

“It was a tough few weeks, but I still have to do my job,’’ Montas said. 

A 33-pitch second inning didn’t help, and he needed 64 pitches just to get through the three innings before being replaced by Albert Abreu. 

“It was a tough situation for him, but he wanted to be out there and compete,’’ manager Aaron Boone said. “He felt he was ready to go. He’s gonna do big things for us.” 

A day after the Yankees were blanked over five innings by Jordan Montgomery, who they had traded to St. Louis on Tuesday, their rotation was in need of a boost from Montas. The Yankees acquired him after the best starter on the market — Luis Castillo — was traded from Cincinnati to the Mariners. 

Castillo, who beat the Yankees on Wednesday at the Stadium, will face them again on Tuesday in Seattle. 

Montas said his shoulder felt fine, but the extended layoff may have impacted his effectiveness. 

“I was a little out of sync with my mechanics and missing my spots a lot,’’ Montas said. “I didn’t have my best delivery. I was all over the place.” 

The one positive he took from the outing was the fact his shoulder held up. 

“My arm feels good,’’ Montas said. “That’s all I care about.” 

The 29-year-old was the Yankees’ most significant acquisition at the trade deadline, part of a package that also brought reliever Lou Trivino from Oakland in exchange for pitching prospects JP Sears, Ken Waldichuk, Luis Medina and minor league infielder Cooper Bowman. 

Montas remains pleased by his new surroundings after leaving the A’s. 

“It’s awesome,’’ Montas said. “This is the best team right now. It’s a great culture and a good group. I’m excited to be here.”