The Yankees’ ace put his team in a bad spot before he recorded an out. Then the ace they didn’t get ahead of the Tuesday trade deadline made their day even more difficult.

Seattle tagged Gerrit Cole for three home runs in the first inning before the newest Mariner, Luis Castillo, took care of the rest, sending the Yankees to a 7-3 loss on Wednesday afternoon in The Bronx.

The Yankees (70-36) dropped the series and lost for the third time in their past four games before heading out on a nine-game road trip that will begin Friday in St. Louis.

Wednesday marked the second straight start in which Cole was rocked in one big inning. Last Friday against the Royals, it was a five-run fifth inning that did him in. On Wednesday, it was a six-run first inning.

“It’s kind of hard to believe how much it just blows up so quick,” Cole said. “I’m trying to be excellent out there and obviously not. Obviously the game’s letting us know we’re not there. So we gotta keep grinding on it. It doesn’t feel good. We let a series get away from us and it’s because of a very poor first inning today. That’s on me.”

The six runs were the most Cole has allowed in any single inning of his career. He settled in from there and did not give up another run while lasting six innings, but the damage was already done — in part thanks to Cole’s counterpart.

Before acquiring Frankie Montas from the Athletics on Monday, the Yankees tried to land Castillo from the Reds. But they found the price to be too high (they did not want to include top prospect Anthony Volpe, The Post’s Joel Sherman reported), which opened the door for the Mariners (57-49) to win the sweepstakes for a haul of prospects.

Making his Mariners debut, Castillo largely cruised, striking out eight over 6 ²/₃ innings. Kyle Higashioka put a slight blemish on his final line, cracking a two-run homer on Castillo’s 109th and final pitch of the day to get the Yankees within 7-3 in the seventh inning.

“When we faced him with the Reds a few weeks ago, he shut us down and was dialed in,” manager Aaron Boone said. “Today, I kind of feel like we had the right at-bats against him and hit some balls right at people.”

Their best chance came in the second inning, when Isiah Kiner-Falefa singled to drive in Andrew Benintendi. But they wasted a chance for a bigger inning when Kiner-Falefa was thrown out at the plate, trying to score from first on a double by Higashioka.

By then, the Mariners had already left their mark on Cole.

After Adam Frazier led off the game with a single and Jesse Winker walked, Eugenio Suarez drilled a three-run homer off a slider for a 3-0 lead.

Carlos Santana made it back-to-back homers when he crushed a 99 mph fastball down the middle 423 feet into the bleachers to make it 4-0.

Three batters later, after a visit from pitching coach Matt Blake and a Bronx cheer following the first out, former Mets prospect Jarred Kelenic roped a changeup for a two-run home run that put the Mariners up 6-0.

“We know they’re a relatively patient team and they pick their spots to swing,” Cole said. “Aggressive pitches just outside the zone weren’t getting flinches and then having to come in the zone, they were anticipating us being in the zone. It’s just like we were just a step behind.”

Cole and Higashioka both said they felt they had the right plan coming into the game, but it ultimately proved wrong.

“I think we’re in accordance with one another,” Higashioka said. “It’s just difficult because I feel like teams, when they face Gerrit, they can be quite different than if they’re facing anybody else. It’s definitely tough to build that into the game plan, but we gotta do it. There’s no excuses. We gotta figure it out.”