Triple-A Toledo placed Horn on its 7-day injured list May 24 due to left elbow inflammation, Chris McCosky of The Detroit News reports.
Horn last pitched for Toledo on May 17 before landing on the shelf a week later, but he may be on the cusp of a return. He began a rehab assignment with Single-A Lakeland on Tuesday and has given up four runs (three earned) on three hits and one walk over 1.2 innings through his first two appearances.