Marlins manager Dan Jennings updated Cosart's status on Wednesday afternoon before a game against the Cardinals. To make roster room for the 25-year-old Cosart, a corresponding move will take place prior to the series finale Thursday.
Cosart, 1-3 with a 4.08 ERA in seven starts, has been on the 15-day disabled list, retroactive to May 14, with vertigo.
"He will be active [Thursday]," Jennings said. "He will be ready to go, ready to pitch. Glad he is ready to go. Glad that he is up here."
Cosart is coming off a rehab-assignment start on Sunday with Double-A Jacksonville, where he gave up one run on three hits in five innings. The glitch was he walked six, but he also struck out six in a game where the organization felt he had 15-20 strikes that were called balls.
Cosart has made 47 big league starts in his career and has never been used in relief.
The Marlins, at least initially, are planning to ease him back into the rotation by throwing out of the 'pen.
"We're talking about him right now, either in the rotation or in the bullpen," Jennings said. "That will be determined. Right now, it looks like he will start out in the bullpen and ease back into it."
Dan Haren will start on Thursday, and Justin Nicolino is getting the nod on Friday in the series opener with the Dodgers. Tom Koehler is set to go on Saturday, and Sunday's starter is yet to be determined.
Jose Urena would be lined up to pitch that day.
Koehler was scratched from a start last Saturday at Cincinnati due to a stiff neck.
Jose Fernandez is still scheduled to start for Double-A Jacksonville on Saturday in his final rehab assignment outing. The Marlins plan on debuting Fernandez on July 2.
Henderson Alvarez (right shoulder inflammation) is throwing 120 feet off flat ground. He is expected to throw off the mound Saturday.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.