Horwitz (hamstring) was diagnosed with a concussion Tuesday after being hit in the helmet with a pitch while on a rehab assignment at Single-A Bradenton, Aiden Stepansky of MLB.com reports.
Horwitz's activation from the 10-day injured list had appeared imminent, but his status is now up in the air as a result of the head injury. The first baseman initially landed on the IL in late June due to a left hamstring strain, but he appears to have made a full recovery from that injury. Horwitz will remain on the IL for now and might need to play more rehab games once he clears concussion protocol.