Middleton was placed on the 15-day injured list with right elbow inflammation after Game 1 of Tuesday's doubleheader against the Dodgers.
Middleton presumably suffered the injury during his last appearance Monday, when he walked one batter during a scoreless inning. It's unclear exactly how long he'll be sidelined, but given it's an elbow injury, an absence beyond the 15-day minimum wouldn't be surprising.