Cosart experienced a tight right hamstring in his May 13 start against the Dodgers at Dodger Stadium. Manager Dan Jennings said the hamstring is fine, but the ear infection created the issue with vertigo.
Koehler, meanwhile, will rejoin the rotation after he was moved a couple of days ago to the bullpen.
Koehler threw three scoreless innings, striking out four against the Braves on Saturday. The right-hander has made eight appearances this season, with seven starts, and he's 2-3 with a 4.29 ERA.
Cosart has been going through a rough stretch. In his last three starts, he has given up 11 runs in 14 1/3 innings (6.91 ERA).
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.