Yankees left-hander Nestor Cortes admitted he was “upset” about being used as a reliever against the Cubs. He said he was surprised by the demotion from New York’s starting lineup but would accept the new role if it was permanent.

Cortes, a 2022 All-Star who has worked exclusively as a starter since the middle of the 2021 season, threw 4⅓ no-hit relief innings at Wrigley Field on Saturday afternoon and paired with starter Clarke Schmidt for a four-hit shutout in the Yankees’ 2-0 victory.

The outspoken Cortes was forthright after the game, calling himself the “workhorse” of New York’s rotation and pointing to the Yankees’ decision to name him their Opening Day starter when ace Gerrit Cole was sidelined with an elbow injury earlier in the season.

“Of course I was upset,” Cortes said. “I felt like I was the workhorse of all the starters here. When Cole went down, they selected me as the opening day starter — not necessarily No. 1, but the opening day starter. I had to change my routine there.

“Now they’re doing it.”

It was the first relief appearance since July 23, 2021 for Cortes (9-10), who lowered his ERA to 3.97 and leads the Yankees with 163⅓ innings pitched this season. He needed just 48 pitches to complete his appearance Saturday, managing one walk and three strikeouts.

“I will never back down from a challenge,” he said. “I will never let my teammates down. I will always do my best, whether I’m happy or not.”

“That’s what I did today. I went out there and proved that I can handle any situation. If that’s my role from now on, then I’ll accept it.”

Cortes is scheduled to start one of the Yankees’ four games against the Boston Red Sox next weekend, but his future role is uncertain as New York is expected to return to a five-man starting lineup.

Cortes, 29, struggled in his last start last Sunday, allowing five runs in four innings in a loss to the St. Louis Cardinals. But he has been outstanding in his previous three starts, allowing just one run while recording 18 strikeouts in 20⅔ innings during that span.

Yankees manager Aaron Boone praised Cortes for his performance out of the bullpen, saying it was “proof that he’s a professional.”

“I’m sure he wasn’t thrilled this time,” Boone said. “But there was still a job to do and he did it brilliantly.”

However, Cortes could ultimately be the Yankees’ underdog, as Schmidt and Luis Gil both return from the injured list and rejoin a rotation that also includes Cole, Carlos Rodon and Marcus Stroman.

“I have no idea,” Cortes said when asked about his role. “This film surprised me, so we’ll see.”

“I know they told me I was starting against Boston, but after that, with a six-man rotation and days off in between, guys are going to get extra days and I don’t think that’s necessarily the right move, so we’ll see.”