Baltimore Orioles first baseman Ryan Mountcastle (6) reacts as trainer Brian Ebel examines his shoulder during the first inning against the St. Louis Cardinals at Oriole Park at Camden Yards. Tommy Gilligan-USA TODAY Sports

The Orioles placed first baseman Ryan Mountcastle on the injured list and recalled outfielder Ryan McKenna from Triple-A Norfolk in a corresponding move, manager Brandon Hyde announced to reporters (via Andy Kostka of the Baltimore Banner). The team’s formal announcement on the injury specifies Mountcastle’s injury as inflammation in the AC joint of his left shoulder.

Mountcastle has missed the past seven games due to discomfort in that shoulder, though stints on the injured list can only be backdated to a maximum of three days. As such, he’ll miss at least the next week. Hyde added that the O’s are hopeful Mountcastle will be able to return when first eligible, though that’ll obviously depend on how his ailing shoulder progresses.

Mountcastle, 26, has been a productive member of a stacked Baltimore lineup in 2023, hitting at a .269/.327/.453 clip with 18 home runs, 20 doubles and a triple in 459 trips to the plate. This will be his second IL stint of the season, as he also missed nearly a month earlier in the summer due to a bout with vertigo symptoms. Mountcastle had looked lost at the plate for a spell heading into that stint on the IL, but he’s been one of the Orioles’ top hitters since returning. In 198 plate appearances between the two IL stints, he’s posted a huge .327/.409/.497 with seven home runs, eight doubles, a 12.1% walk rate and a manageable 21.1% strikeout rate.

With Mountcastle shelved at least a week, the O’s will likely lean on Ryan O’Hearn at first base and outfielder Anthony Santander at designated hitter. Newly recalled top prospect Heston Kjerstad, the No. 2 pick in the 2020 draft, could see time in right field, at first base or at designated hitter as well.

The Orioles have already clinched a postseason spot — their first since 2016 — so there’s little reason to rush Mountcastle back. That said, the team would surely prefer to get a look at him in a game setting, if possible, before determining its postseason roster. The obvious hope is that the downtime for Mountcastle will allow him to recover enough to reclaim his spot in the heart of the team’s batting order. Baltimore’s lineup is one of the deepest in the league, but potentially losing a bat of Mountcastle’s caliber would be a notable blow to any team’s offense. Mountcastle is also the team’s top performer against left-handed pitching; he’s torched southpaws with an absolutely mammoth .340/.396/.660 slash line in 2023.

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