HOUSTON — Ryan Mountcastle is headed to the 10-day injured list, Orioles manager Brandon Hyde said Wednesday, ruling the first baseman out for at least the next week due to left AC joint inflammation.

Mountcastle left last week’s series finale against the St. Louis Cardinals with shoulder discomfort and has missed seven games, including Wednesday’s series finale in Houston against the Astros. Mountcastle said he felt a pop in his shoulder and was “just praying for the best.”

At the time Hyde said Mountcastle wasn’t an injured-list candidate, and the manager reiterated that throughout Mountcastle’s time on the sideline. But Mountcastle is now headed to the IL because his swing hasn’t progressed the way Baltimore expected.

“Tried to swing today, didn’t go as well as we were hoping,” Hyde said. “It’s not a setback. It was going really well with his progression and he’s feeling better. He tried to swing the other day, was a little bit sore. We were hoping this session today was going to be an improvement from that last one, and it just wasn’t.”

Hyde said he hopes Mountcastle will be ready to play once his injured-list stint expires Sept. 27. According to MLB rules, the maximum number of days a club can backdate an injury is three, regardless of how many games that player missed.

Outfielder Ryan McKenna is en route to Houston to join the team.

This is Mountcastle’s second trip to the injured list this season. He missed about a month in June and July for vertigo. Mountcastle had begun to find a rhythm at the plate again, but that progress has been stifled once more.

Mountcastle is hitting .269 with a .780 on-base-plus-slugging percentage. When healthy, he and Ryan O’Hearn have shared the duties at first base and designated hitter.

“He’s really down about it,” Hyde said. “It’s disappointing for him. He was hoping he’d be back in a few days, and just wasn’t — the treatment that he’s getting and how hard he’s working to get back — just disappointed that it wasn’t feeling good in his batting practice today in the cage.”

O’Hearn has been on a tear, raising his average to .311 with an .859 OPS. He went 5-for-5 in Monday’s series opener against Houston and followed that with two hits Tuesday, including a two-run homer.

O’Hearn will likely take the bulk of the first base duties while Mountcastle is out, but any left-on-left matchups might prompt Hyde to switch. Among the other options are outfielder Anthony Santander, utility infielder Ramón Urías and catcher James McCann. Outfielder Heston Kjerstad has also briefly played first base in the minors.

The injury comes with under two weeks remaining in the regular season. Mountcastle could return during the final homestand of the season, when the Washington Nationals and Boston Red Sox visit Camden Yards.

Should the Orioles secure the American League East title, they’ll also earn a bye through the wild-card round of the postseason. That could provide Mountcastle more time to recover.

andy.kostka@thebaltimorebanner.com