Corey Kluber walked three batters in the first inning of his latest start and his continued struggles could cost him a spot in the Red Sox starting rotation. Brandon Sloter/Associated Press

SAN DIEGO — Garrett Whitlock (right elbow ulnar neuritis) tossed 4 2/3 scoreless innings in a rehab game for Triple-A Worcester on Sunday. The Red Sox will activate him from the 15-day injured list Saturday to start against Arizona.

That means another cut is coming soon but not immediately.

Manager Alex Cora and Chief Baseball Officer Chaim Bloom chose to demote Nick Pivetta to the bullpen Wednesday when the Red Sox returned to a five-man rotation after a few weeks of using six starters.

Corey Kluber, who lasted only 2 1/3 innings in Boston’s 7-0 loss to the Padres on Sunday at Petco Park, could be the next starter cut from the rotation with Whitlock returning. But the two-time AL Cy Young winner will make his next start Sunday in Arizona, Cora confirmed Kluber’s start against the Padres.

Will the Red Sox use a six-man roster for the foreseeable future?

“For the foreseeable week,” Cora replied.

That means all six starters will make their next start.

Tanner Houck will start Monday when Boston opens a three-game series in Anaheim. Brayan Bello will pitch Tuesday and James Paxton will pitch Wednesday. Chris Sale will start the first game of a three-game series in Arizona on Friday. He will be followed by Whitlock and Kluber.

Then it’s decision time.

Kluber’s control issues are a mystery. He led the league with the fewest walks per nine innings (1.2) last year. He walked two or more batters in only three of his 31 starts.

But the veteran right-hander (2-6, 6.26 ERA) has walked multiple hitters in five of his nine starts this season. He walked three batters in the first inning Sunday, including one with the bases loaded.

“That not him,” Cora said. “We’ve been battling with this early in the season. Something we’re trying to correct. But today the walks put him in a bad spot.”

Cora and Kluber don’t have an answer for Kluber’s control issues.

“If I knew, put it in a bottle, drink it and throw strikes over there, to be honest with you,” Cora said. “We’re working hard with him because we know when he’s around the zone, it’s tough to hit.”

Kluber said it’s frustrating for him and everybody else.

“Just doing a poor job of finding the correct adjustment to make,” Kluber said.

“Trying to work through different things, adjustments we’ve talked about in between starts and in the past,” he added. “Just can’t find one to click right now.”

MICHAEL WACHA signed a creative deal with San Diego that amounts to a four-year, $26 million guarantee with options that can be picked up by both the team and player. He pitched last season for Boston on a one-year, $7-million contract.

Did the Red Sox try to find a way to keep the 31-year-old righty?

“I think we tried but they had other thoughts,” Cora said. “This is agent stuff. We did our best, I guess. But they were looking at something else. This kid, he posted last year. He pitched well. Good teammate, preparation. One of the best I’ve had. Up there with Rick (Porcello). The same way in the clubhouse helping people out. And you could see him on the computer getting ready for his outings. He was really good for us. He was good against the (AL) East. It’s a guy I really respect.”

Wacha struggled in April but he has pitched well in May. He’s 2-0 with a 0.47 ERA (19 innings, one earned run), 0.63 WHIP and .094 batting average against in three starts this month. He went 2-1 with a 6.75 ERA (25 ⅓ innings, 19 earned runs), 1.58 WHIP and .311 batting average against in five starts in April.

“The last two outings, he’s made some adjustments and that’s what he does,” Cora said. “I know he’ll be prepared for us.”

Wacha pitched six scoreless innings against the Red Sox in San Diego’s 7-0 win on Sunday.

Wacha would have liked to return but it doesn’t appear Boston got anywhere close to re-signing the righty.

“I don’t know. I guess you might have to ask Chaim (Bloom) about that one,” Wacha told MassLive in April when asked by Chris Cotillo why nothing worked out with the Red Sox. “They were showing interest.

“They reached out, for sure,” the righty added. “But just wasn’t able to get something worked out with them. It is what it is, I guess.”

THE RED SOX placed left-handed reliever Richard Bleier on the 15-day injured list because of left shoulder inflammation.

They recalled lefty Brennan Bernardino from Triple-A Worcester. Bernardino already is here. He joined the team Sunday as part of the taxi squad. He has a 3.65 ERA (12 1/3 innings, five earned runs) in 11 appearances this season for Boston.

Bleier has a 5.85 ERA (13 earned runs, 20 innings) in 19 relief appearances this season. Lefties have gone 12-for-28 (.429 batting average) with three homers and one double against him.