BALTIMORE—The Orioles are getting ready for a huge, four-game series against the Tampa Bay Rays, one that will help decide the winner of the American League East.

They may have to face the Rays, whom they lead by two games, without first baseman Ryan Mountcastle.

Mountcastle left Wednesday night’s game in the third inning with left shoulder discomfort. During his first-inning at-bat against former Oriole Drew Rom, Mountcastle fouled a ball off, walked and was met at first base by head athletic trainer Brian Ebel.

“Not great, just got to get some imaging done tomorrow. Not sure what happened,” Mountcastle said. “It doesn’t feel good, just praying for the best.”

“I saw him wince after the foul ball,” manager Brandon Hyde said. “He’s day-to-day right now. Hopefully, he’s better tomorrow. I’m just not sure.”

Mountcastle missed a month with vertigo earlier this season and doesn’t want to miss crucial games.

“Took a swing, popped and just is what it is. I don’t know what it is,” Mountcastle said. “I just played a little defense, tried to take a swing in the cage and definitely, it wasn’t ready to go.”

Ramón Urías pinch-hit for Mountcastle in the third inning and went to first. Ryan O’Hearn pinch-hit and played first in the last two innings.

“Obviously, you don’t want to lose anybody right now, especially Ryan,” Hyde said. “We’ll see. Hopefully, he’s OK tomorrow.”

After watching the Rays beat the Minnesota Twins, 5-4, on their clubhouse televisions before Wednesday night’s game, the Orioles supplied no offense and wasted an outstanding start by Kyle Gibson, losing 1-0 to the St. Louis Cardinals before an announced crowd of 14.442 at Camden Yards.

As a result, the Orioles, who dropped two of three to the Cardinals, will begin their series against Tampa Bay two games ahead of the Rays in the AL East. Their magic number to clinch a playoff spot remains at four. They had won their previous seven series.

The Orioles removed Heston Kjerstad, their power-hitting outfield prospect, from Triple-A Norfolk’s game for a pinch-hitter after Mountcastle left the game. Kjerstad, who has played some first base for Norfolk and Double-A Bowie, is the 24th^–rated prospect by MLB Pipeline.

The Cardinals started Rom, who, along with Triple-A infielder César Prieto, was traded to St. Louis for starter Jack Flaherty on August 1st.

Rom (1-2) threw 4 2/3 hitless innings before Jorge Mateo had an infield single. He was removed after Anthony Santander’s double with one out in the sixth. He struck out seven and walked three.

“We didn’t square many balls up so give him credit. He pitched really well,” Hyde said. “We didn’t do much.”

After Rom left, Casey Lawrence, Giovanny Gallegos, John King and Ryan Helsley held the Orioles (91-54) scoreless over the last 3 2/3 innings.

Gunnar Henderson tripled with two outs in the ninth, but Aaron Hicks popped to short to end the game. Helsley recorded his ninth save as the Orioles were shut out for the eighth time.

Richie Palacios, the Towson University star who homered twice in Tuesday night’s game, homered in the fourth inning against Kyle Gibson, who pitched brilliantly but got no run support.

“I try to be pretty critical of myself, but man … Obviously I’d like to have that pitch back,” Gibson said. “Not going to beat myself up over one pitch. Very rarely are you going to lose games, 1-0, on a team like this that is really good. They just pitched a little better tonight.”

Gibson (14-9) allowed one run on three hits in 6 2/3 innings.

“Any series is tough, but I still think we played pretty good baseball,” Gibson said. “I think the last thing we need to do right now is make too much about one loss and losing a series.

“Does it stink that Tampa won? Sure, but we’re two games up, and if we play really good baseball the next three weeks, we’ll be fine. You never want to lose a series, but shoot, it’s going to happen at some point. You’re not going to win the rest of your games. We’d like to, but it’s not going to happen.”

Notes: Garrett Stallings (7-4) allowed one run on three hits in seven innings in relief of Austin Voth as Norfolk defeated Memphis, 3-2. Centerfielder Colton Cowser hit his 15^th home run. … Rightfielder Dylan Beavers drove in three runs and hit his second home run as Double-A Bowie beat Harrisburg, 7-4. Centerfielder Jud Fabian, catcher Silas Ardoin and designated hitter Connor Pavolony also homered. … Kyle Bradish (11-6, 3.03) will face Tampa Bay’s Aaron Civale (7-3, 2.96) on Thursday night. The game will start at 7:15, and will be televised exclusively on FOX.