The 24-year-old was scratched from his start on Thursday at Pittsburgh due to what the team called a sore left oblique.
After throwing off the mound on Saturday at Cincinnati, Cosart said the issue was more related to lower back stiffness than an oblique.
Either way, the right-hander is ready to go.
"I think I'm over it," Cosart said.
The Marlins acquired the right-hander from the Astros on July 31, and he made his Miami debut the next day. Cosart went 5 1/3 innings and suffered the loss Aug. 1 to the Reds at Marlins Park.
During the game, the former American League pitcher -- with limited at-bats -- legged out an infield single. On the play, he tweaked the left side of his lower back. All along, he claimed it wasn't serious. The team played it carefully and opted to have him miss one start.
"We did the right thing backing him off and making sure he is 100 percent healthy," manager Mike Redmond said.
Prior to the trade, Cosart was 9-7 with a 4.41 ERA in 21 starts for the Astros.
Cosart did make some light throws off the mound at Pittsburgh on Thursday, but he let it loose on Saturday.
"I let it go pretty good today," he said. "I'm good to go. I'm excited."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter