MIAMI — With Tyler Mahle’s pending return to the rotation, the Giants faced a coming logjam of starting pitchers. The odd man out, despite his objections: Adrian Houser.

“I mean, I’m not going to be happy with it, but I understand,” Houser told The California Post after manager Tony Vitello delivered the tough news Saturday. “I signed here to be a starter. I didn’t sign here to be in the bullpen.”

Yet, that is precisely where the 33-year-old right-hander will find himself only three months into his Giants tenure, at least for the time being. In 14 starts, Houser was 2-6 with a 5.73 ERA with a 1.573 WHIP, the highest of his career and the third-worst mark of any starter with at least as many innings.

“He wants the same thing as everybody else,” Vitello said. “He wants to do better.”

Houser, in particular, has struggled against lefties and in the first inning. Left-handed hitters are responsible for nine of the 12 home runs Houser has allowed while batting .339 with a 1.042 OPS, compared to right-handers’ .218 average and .540 OPS.

In his last start, Houser served up a 473-foot home run to Drake Baldwin in the first inning, raising his ERA in the first frame to 9.64. His ERA in the fifth, when the lineup typically turns over for a third time, swells to 12.46. But in between, he owns a 2.79 mark.

“He’s really found a rhythm in the middle of outings,” Vitello said. “The first inning has given him trouble. Third time through the order has given him trouble. Those are things he’s done well before in the past.”

That track record, most recently featuring a 3.31 ERA in 21 starts last season for the Rays and White Sox, led president of baseball operations Buster Posey to award Houser a two-year, $22 million contract to round out their starting rotation with Mahle, another free-agent addition.

Mahle hasn’t been any better — 1-7 with a 6.04 ERA in 11 starts — but could be a trade piece at the deadline. He is only under contract for the rest of this season at a rate of $10 million.

Houser, on the other hand, will attempt to convince Vitello and the Giants brass that he belongs in the rotation. Of his 187 career appearances, 139 have come as a starting pitcher.

“I proved last year that I’m a starter and I’ve done it in years past, too,” Houser said. “At the end of the day, it’s still the same goal – you’ve got to get outs. I’m gonna do everything I can to get back in the rotation because I signed here to be a starter, not a bullpen guy.”

Houser talked with The Post following a “good day of catch play,” where he was getting hands-on advice from director of pitching Frank Anderson. The team believes his struggles against left-handers has more to do with pitch selection than mechanics.

“The sinker’s been getting damaged a little bit, and that’s kind of been the root cause,” pitching coach Justin Meccage told The Post. “So a little more four-seam usage. Really, a true mix of all four pitches. And moving the fastball around the different quadrants.”

Because of his release point, lefties tend to see the ball better out of Houser’s hand than a typical righty, Meccage said. Too predictable of a pitch mix makes it even easier to tee off on.

“So you’ve got to disguise pitches to create an unpredictable look,” he said. “Make that [swing] decision really hard as long as possible. That’s where the sequencing comes into play.”

Houser has started to incorporate his four-seamer more, but teams still have stacked lefties at the top of the lineup, leading to his troubles in the first inning.

A simple solution, it would seem, would be to use a left-handed opener in front of Houser if and when he returns to the rotation. The idea has been bandied about among Giants coaches, but Houser said he hasn’t been approached about it. 

He would be about as receptive to it as he was about his new assignment in the bullpen.

“I’m not big on it,” Houser said. “I like to stay in my routine.”

So much for that routine, at least for now.

Houser, having only thrown one inning before his last start was interrupted by rain, will be available in the bullpen beginning Saturday, Vitello said. How he will be used remains to be determined, though they expect to need some length behind Mahle in his return Wednesday.

Given Houser’s objections and his status as a player Posey hand-picked to fill a role that has suddenly changed, it made for a test for how the rookie manager would deliver hard news.

“There’s no real easy way to tell somebody they’re being demoted or moving to the bullpen or someplace they don’t want to go to,” Houser said. “He handled it the best he could.”