Reliever Pedro Baez is injured in Dodgers’ 5-4 loss to Marlins

Right when their constantly transforming rotation was starting to take a more definitive shape, the Dodgers found themselves uncertain of what their bullpen would look like in the coming weeks.

In the eighth inning of a 5-4 defeat to the Miami Marlins on Wednesday night, right-hander Pedro Baez walked off the field at Dodger Stadium with a trainer by his side and discomfort in a chest muscle.

“It could be short,” Manager Don Mattingly said of Baez’s anticipated absence. “But usually something like that is a little longer than that.”

Baez, who has a 1.76 earned-run average, is expected to undergo an MRI exam Thursday before the Dodgers open a four-game series against the Colorado Rockies.

Right-hander Chris Hatcher sounded concerned. Baez replaced Hatcher with the bases loaded and struck out J.T. Realmuto with a 97-mph fastball to end a three-run seventh inning for the Marlins.

“Baez has been pretty much a shutdown guy for us,” Hatcher said. “To see a guy coming out of the game that has been so impactful, you hope it’s something minor, a stinger, a tweak. But to be honest, I don’t know what it is and I don’t think he does, either.”

The impending return of closer Kenley Jansen could help the Dodgers absorb the loss of Baez.

Jansen appeared in the last scheduled game of his minor league rehabilitation assignment, pitching a perfect inning for Class-A Rancho Cucamonga.

The closer is recovering from a foot operation he underwent before the start of spring training.

The Dodgers had originally planned to activate Jansen on Saturday.

“Saturday is definitely not a hard date,” Mattingly said in the wake of Baez’s premature exit.

Regardless of when Jansen returns, the Dodgers are certainly a better team with Baez than they are without him.

A 27-year-old converted third baseman, Baez has established himself in the first five-plus weeks of the season as one of Mattingly’s most dependable late-inning options. He is often cited as one of the primary reasons the Dodgers bullpen has unexpectedly ranked among the best in baseball.

Catcher Yasmani Grandal said Baez called him to the mound after giving up a one-out double to Dee Gordon in the eighth inning.

“He said he felt something under his armpit,” Grandal said.

Grandal signaled to the bench, prompting trainer Stan Conte to visit the mound.

Baez’s night was over.

“Hopefully, we detected it early enough,” Grandal said.

If Grandal was concerned about Baez, he was also optimistic about starting pitcher Carlos Frias, who pitched into the seventh inning in his third start of the season.

Frias was charged with three runs and six hits over six innings, which was enough to get Mattingly to say Frias would remain in the rotation.

The Dodgers have used nine starting pitchers this season, but Mattingly has said in recent days that he would like to stabilize the rotation instead of continuing to shuttle pitchers back and forth from triple-A Oklahoma City.

Mike Bolsinger, who started in the Dodgers’ victory over the Marlins on Tuesday night, will start the series finale against Colorado on Sunday.

“We’re getting to the point where we have to settle in and get on track with our five guys,” Mattingly said.

Frias, 25, said he gained 10 pounds over the off-season working out with a personal trainer in his native Dominican Republic. The increase in weight has resulted in an increase in fastball velocity, as he now regularly throws in the mid-to-high 90s.

In five games this season, including three starts, Frias has posted a 2.89 ERA.

What pleased Grandal the most Thursday was how Frias used his off-speed pitches.

“He’s got room to grow,” Grandal said. “I think that’s going to happen the more he pitches, the more he starts.”