ST. LOUIS — Slipping of late but still holding a slight edge for the best record in the American League, the Yankees took action last Monday ahead of the Major League Baseball trade deadline. They sent two of their top 10 prospects and two other minor league players to the Oakland Athletics in exchange for the relief pitcher Lou Trivino and the starting pitcher Frankie Montas.

Although Trivino, a right-hander, had been underperforming this season compared with his career norms, he reinforces a bullpen undermined by injuries. He has already made four appearances for the Yankees. But Montas, the centerpiece of the trade and the type of rotation help that other contending teams had also been seeking, had yet to made his Yankees debut.

Because of the death of his mother-in-law, Montas did not join the Yankees until Saturday evening, meeting them in St. Louis. He huddled with catcher Jose Trevino at the team hotel. And a day later, he took the mound as a Yankee for the first time. But even he could not stop the team’s worst skid of 2022.

Montas had his worst start of the season, allowing six runs over three innings Sunday and later admitting that his delivery was “all over the place.” The 12-9 loss to the surging Cardinals capped the first three-game sweep of the Yankees this season and extended the team’s season-worst losing streak to five.

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

On July 8, the Yankees were on a pace (118) to break a M.L.B. record for wins (116) in a 162-game regular season. They have sputtered since, going 9-16. The Yankees (70-39) still maintain a sizable lead in their division, the A.L. East, but cracks have been showing and they have lost ground to the Houston Astros (70-40) for the top A.L. seed in the postseason.

Against the Cardinals (60-48), who are in first place in the National League Central and have now won seven straight games, Montas’s command was inconsistent. In the second inning, with the Yankees leading, 4-1, he walked the first two batters. Dylan Carlson and Paul Goldschmidt each drove in a run — with another walk in between — to bring the Cardinals within one run. Then Nolan Arenado smashed a go-ahead, three-run home run that earned him a curtain call.

Right fielder Aaron Judge tied the score with a two-run double in the fifth inning, but the Yankees’ bullpen squandered that in the bottom half of the frame by allowing three runs.

“We’re definitely going through it right now and frustrated with how we’ve played,” said infielder D.J. LeMahieu, the lone Yankee to homer Sunday. “When we get a good pitching game, we don’t hit, and vice versa.”

Perhaps a reason for Montas’s struggles was his irregular schedule of late. A right-hander, he returned July 21 after missing nearly three weeks with inflammation in his throwing shoulder. He started again for Oakland on July 26, reaching 78 pitches, then was traded to New York. But then came the family death that delayed Montas’s arrival and affected his workload, which, according to Manager Aaron Boone, was still being built back up after the injury anyway.

While Montas was on the bereavement list, Sam Briend, the team’s director of pitching, flew to Arizona to meet with him and oversee his throwing, including a bullpen session, said Matt Blake, the team’s pitching coach.

“We didn’t want him to be out on his own for four or five days and then come and start, so Sam went, kind of got eyes on him, talked through what the expectations were, and gave us a download of what he does in his routine and everything,” Blake said.

Boone added: “We got about as good a week as you can considering the circumstances.”

Ahead of the trade deadline, the Yankees added the All-Star outfielder Andrew Benintendi, a left-handed contact hitter who helps further balance the lineup and weather the absence of Giancarlo Stanton (left Achilles’ tendinitis) and the struggles of Aaron Hicks (.226 batting average); the right-handed reliever Scott Effross, who wracks up strikeouts throwing sidearm; Trivino; and Montas.

Montas, 29, fortifies a rotation that has dealt with some struggles (Domingo German has a 5.09 E.R.A. in four starts since his return from a shoulder injury) and that will be without Luis Severino (right latissimus dorsi strain) until mid-September.

But the Yankees also subtracted from their rotation, surprisingly sending the 29-year-old left-hander Jordan Montgomery — who was drafted by the Yankees in 2014, had been pitching solidly (3.69 E.R.A.) and was under team control next year — to the Cardinals for Harrison Bader, a 2021 Gold Glove-winning center fielder who is on the injured list until perhaps September. Although Bader was hitting .256 this season and has been out since late June with plantar fasciitis, he can help shore up the Yankees’ weakest defensive outfield position.

(Calling it emotional and weird to face his former teammates so quickly, Montgomery tossed five scoreless innings against the Yankees in a 1-0 win Saturday.)

Although the baseball industry saw Montas as an upgrade over Montgomery, General Manager Brian Cashman recently said that he did not acquire Montas to then dispatch Montgomery. He said trading for Montas, who will be a free agent after the 2023 season, and swapping Montgomery for Bader were done with the goal of “how can we best be flying high with the best of our abilities when it counts the most in October and what gives us the most amount of quality choices.”

Blake said Montas was similar to Severino, “a bulldog on the mound who attacks you with power.” He added later: “For us, it’s a mid- to upper-90s right-handed pitcher with a full arsenal who can get righties and lefties out. It just fits right at the top of our rotation and gives us another guy that we feel confident going into a postseason with.”

Montas, who originally signed out of his native Dominican Republic with the Boston Red Sox, found his footing with the Athletics after being traded several times. In six years in Oakland, Montas was 35-30 with a 3.70 E.R.A. over nearly 538 innings, was suspended 80 games in 2019 for a performance-enhancing drug and tossed over 180 innings in a season just once (in 2021, with a 3.37 E.R.A.).

Before joining the Yankees, Montas had a 3.18 E.R.A. in 104⅔ innings this season. His first impression didn’t go well, but as they plan for October, the Yankees will need Montas to round into form.

“This is the best team right now, you know?” he said. “They have a good culture and a really good group right here. I’m really excited to be here.”