Alvarado was removed from Tuesday's game against the Cubs with mid-back spasms, Charlotte Varnes of The Athletic reports.
Alvarado gave up one earned run on two hits and a walk in one-third of an inning before his back forced him to come out of the game. It's unknown if he will have to spend time on the IL, though the Phillies could sit him down for a few days as a precaution, regardless of the severity of his injury.