The Cardinals placed Middleton on the 15-day injured list Tuesday with a right forearm strain, Jeff Jones of the Belleville News-Democrat reports.
Middleton has been shut down from throwing since March 16, and although he's slated to begin his throwing progression soon, his absence will officially extend into the regular season. An exact timetable for the right-hander's return is unclear, but a better picture should emerge as he gets further along in his recovery process.