Alvarez made just two starts before experiencing shoulder/elbow discomfort during his April 12 outing against the Rays. An MRI revealed inflammation.
Alvarez threw 76 pitches in a rehab assignment start for Class A Jupiter on Tuesday.
In his two starts, the right-hander is 0-2 with a 4.50 ERA.
Koehler is one of the organization's most versatile and durable pitchers. The right-hander is 2-3 with a 4.62 ERA in seven outings. He has experience working out of the bullpen.
Miami's bullpen has labored, and Koehler, who throws in the mid-90s, could maximize his velocity in the upper 90s by pitching in a setup role. He has made 63 career starts, and is 17-24 (4.17 ERA). Koehler has logged 19 2/3 innings in relief in 13 games, posting a 4.12 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.