Slowey exited with a left lat strain. Redmond called it an intercostal issue after the game.
The right-hander will be re-evaluated. The club believes it isn't too severe, but whether Slowey will make his next scheduled start is uncertain.
Slowey said he felt a twinge on his last pitch of the fourth inning, a slider to Domonic Brown, which resulted in a strikeout.
"I felt it grab a little bit underneath," Slowey said. "I went out and [threw] mostly fastballs in the fifth. I was feeling for it a little bit, and came back in after the fifth and the training staff just asked if something cramped up. I said I thought so. My hope was to go out and throw the sixth and beyond. My pitch count was pretty low. Again, I didn't want to go out there not give my team a chance to compete."
After taking a few warmup pitches, Slowey signaled to the dugout, and he was replaced by Wade LeBlanc, who entered in a 1-0 game.
Slowey gave up a solo homer to Delmon Young in the fourth. It was the lone run he allowed in five innings, as he struck out six while allowing six hits and throwing 69 pitches.
Slowey said he was checked by Miami's medical staff at the ballpark, and the initial prognosis is it is not severe. The best case scenario is it is more of a cramp than a strain.
If the discomfort isn't much better by Thursday morning, Slowey may get an MRI.
The Marlins are off Thursday before opening a road trip in Chicago against the White Sox on Friday.
"The hope is I will wake up [Thursday] and say, 'It's sore. It's not cramped up any more, and we'll just go day to day,'" Slowey said.
Slowey has been one of the Marlins' steadiest starters. The right-hander, who formerly was with the Twins, has a 3.30 ERA in 57 2/3 innings.
A non-roster invitee in Spring Training, Slowey pitched his way onto the Opening Day roster and was Miami's No. 2 starter.
"He just felt it in his side," Redmond said. "He thought that maybe he was able to work through it. We wanted to get him out of there. He seems to think he will be alright, hopefully, for his next start, but we'll evaluate him over the next couple of days to see how he is."
The Marlins opened the season with two projected starters -- Nathan Eovaldi and Henderson Alvarez -- on the disabled list with right shoulder inflammation. Both are currently on the 60-day disabled list.
If Slowey is to miss a start, LeBlanc is an option on the active roster to fill is as a spot starter. Jacob Turner, who was in the rotation at the end of 2012, is another possibility. Turner is at Triple-A New Orleans.