ST. LOUIS — Trading away Jordan Montgomery has already hurt the Yankees.

The left-hander, making his first start for the Cardinals, tossed five scoreless innings, but the Yankees have bigger concerns after their 1-0 loss Saturday night.

They have lost four games in a row for the first time this season, as well as five of six overall. And they have dropped two straight series, something they hadn’t done previously in 2022.

While their record remains an impressive 70-38, they have gone 6-9 since the All-Star break and they have the Astros on their tails for the best record in the American League. Houston is just a half-game back at 70-39.

“We’re not excited about it,’’ Aaron Judge said. “But we’ve still got a great ballclub. … We lost two series [in a row]. So what? It’s time to move on.”

The latest defeat came in front of the largest crowd of this version of Busch Stadium — 48,581 — as the Yankees finished with just a pair of hits, and none after the third inning.

“We’re losing close games that we were winning all the time [earlier in the season],’’ manager Aaron Boone said. “Nobody’s happy about it, but we’re not gonna panic or worry about it. … We’ll be fine.”

Facing Montgomery in his first start since they traded him for injured outfielder Harrison Bader on Tuesday, the Yankees couldn’t touch their former teammate. Montgomery allowed just two hits and a walk in five shutout innings before being removed due to cramping in an 83-pitch outing.

“It was a little funky,’’ Judge said of going up against Montgomery. “It brought me back to when I faced him in spring training at-bats. Seeing him in red is a little different.”

“I was pretty nervous especially with how hot it was,” Montgomery said after the outing. “The humidity is definitely different from New York. My body started cramping up. I gave the team five innings.”

Domingo German, pitching in Montgomery’s spot in the rotation, with newly acquired Frankie Montas scheduled to make his first start as a Yankee on Sunday, was nearly as good. German allowed one run on four hits and no walks in his five innings against the Cardinals, who moved into first place in the NL Central with their sixth straight win.

The Yankees threatened in the top of the first, but came away empty.

After Judge singled to left with one out and Josh Donaldson walked, Montgomery received a visit from St. Louis pitching coach Mike Maddux. The lefty then went to a full count against Gleyber Torres before Torres hit a hard grounder to third, which was turned into a double play by Nolan Arenado.

The Yankees hardly sniffed a scoring chance after the first and the Cardinals got to German in the bottom of the inning.

After he retired the first two batters, Paul Goldschmidt doubled to left and Arenado finished a 10-pitch at-bat with an RBI single to left.

German nearly gave up another two-out run in the third.

Nolan Gorman singled to center and Goldschmidt doubled to right-center, but center fielder Tim Locastro started a perfect relay to Gleyber Torres, who fired a strike to Kyle Higashioka at the plate.

Montgomery left with a one-run lead, replaced by Jordan Hicks, who tossed 1 ¹/₃ shutout innings.

“We played good baseball and we got rewarded for it,” Montgomery said. “It definitely feels good. This team is playing good. I’m glad I could come in and keep it going.”

The Yankees barely had any scoring opportunities after the first inning, and they got just one more hit the rest of the way.

With one out in the top of the seventh, former Cardinal Matt Carpenter greeted lefty Genesis Cabrera with a shot to right-center that was tracked down by Dylan Carlson at the warning track for the second out.

Isiah Kiner-Falefa led off the eighth inning with a walk. After Andrew Benintendi pinch-hit for Locastro and popped out, Higashioka ripped a liner to shallow right, but Lars Nootbaar charged in and made a terrific diving catch.

Another former Yankee, Giovanny Gallegos came in for the ninth and got Judge to fly out to center, struck out Donaldson looking and whiffed Torres swinging as he finished it for the red-hot Cardinals.