Windler, who was selected by Cleveland in the first round of this year's NBA draft, experienced discomfort in his leg following some recent team workouts. The Cavs said imaging tests revealed a tibial stress reaction, which is a precursor to a fracture.
He'll begin treatment, and the Cavs say he'll be out at least one month. That could put him in jeopardy of missing the start of the regular season.
The 6-foot-8 Windler averaged 21.3 points and 10.8 rebounds as a senior at Belmont. The Cavs liked his versatility and range -- he made 43% of his 3-pointers last season -- and selected him with the No. 26 overall pick.
Cleveland opens training camp Tuesday under new coach John Beilein, who was previously at Michigan. The Cavs' first preseason game is Oct. 7.