Garnett returned Friday to Boston after injuring the knee on the team's West Coast trip. Celtics spokesman Jeff Twiss says Saturday that team doctor Brian McKeon diagnosed the muscle strain after tests and an MRI.
"Rest and ice for KG," Celtics president Danny Ainge told The Boston Globe.
Garnett injured his knee while going up for an alley-oop late in the first half of Boston's 90-85 loss to the Utah Jazz on Thursday. He landed gingerly and immediately motioned to the bench for a substitute.
After the game, the team decided to send him back to Boston as a precaution when the soreness in his knee persisted. The Celtics play in Phoenix on Sunday.
Information from The Associated Press contributed to this report.