Alvarez opened the season on the DL with right shoulder inflammation. He made his first start with Miami on Thursday.
After opening the season in the bullpen, Koehler was moved to the rotation. In all, he's made 16 appearances, with 10 starts, posting a 1-5 record with a 4.70 ERA.
"Tom will go down there and be in the rotation," Marlins manager Mike Redmond said. "I thought he did a good job while he was here. He gives us some flexibility where he could start or pitch out of the bullpen. He got some valuable experience. He's a guy who has got a great arm."
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter