OAKLAND, Calif. — The Yankees’ no-longer-secret weapon is back in the bullpen.

Ian Hamilton was activated off the injured list on Wednesday after missing six weeks with a groin strain, returning to bolster a relief corps that has been the best in baseball with a 2.83 ERA.

To open a roster spot for Hamilton, the Yankees optioned Jhony Brito to Triple-A Scranton/Wilkes-Barre after he turned in another solid start on Tuesday, throwing 5 ²/₃ innings in a loss to the Athletics.

The Yankees will have to call up a spot starter on Sunday in St. Louis — likely to be Randy Vasquez — before Carlos Rodon is expected to return and fill the rotation spot the next time through on July 7 against the Cubs.

Hamilton, who earned a spot on the roster with a strong spring training as a non-roster invitee, had posted a 1.23 ERA in 16 appearances before getting hurt.

The right-hander struck out 30 and walked nine across 22 innings, working his way into higher-leverage situations, including picking up his first career save on May 6 against the Rays.

“It’s really good to get him back,” manager Aaron Boone said before Domingo German pitched a perfect game in the Yankees’ 11-0 win over the A’s on Wednesday night. “He was doing such a nice job for us, obviously from the start of spring training when he jumped on our radar, kicked his way onto the team with how well he threw and then he carried it right into the season. And doing all kinds of roles — everything from pitching important back end of the game leverage to giving us length on given days. He’s done a good job of getting both righties and lefties out.”

With the return of Hamilton, the Yankees will go with a nine-man bullpen until Sunday, when they will likely have to send a reliever out for Vasquez.

Tommy Kahnle was charged an automatic ball on Tuesday night for running out of time on the pitch clock after dropping the baseball while getting a sign.

Kahnle raced to pick it up and get back on the rubber, but he did not begin his delivery in time.

Boone argued, to no avail, that the clock should have been reset after Kahnle inadvertently dropped the ball.

Boone continued his conversations about the incident on Wednesday with the league.

“I don’t know about clarity, but I definitely had some long conversations about it today,” Boone said. “I’m still adamant, I think they should have reset [the clock]. One of the things that got him in trouble was with two seconds, asking for [a new] ball. It’s always umpire discretion, but when something happens like it falls out or a dropped ball, if it’s not deemed a guy doing it on purpose to try to buy time — which I don’t think anyone would suggest that’s what happened [Tuesday] night, it was simply dropping the ball. To me, that should have been grounds for a reset.”

Giancarlo Stanton started his second game in the outfield on Wednesday since coming back from the IL, playing the spacious right field at Oakland Coliseum.

Stanton went 2-for-3, including a solo home run, with three RBIs before Oswaldo Cabrera ran for the slugger following a walk in the seventh inning.

Boone said Frankie Montas (shoulder surgery) is doing well in his throwing program, but has not yet thrown off the mound.