Spartans coach Tom Izzo told reporters Friday that Watts has been dealing with leg issues and described the injury as a stress reaction on a bone. Izzo said Watts' availability next week could also be affected.
Watts has started the first eight games this season for the 11th-ranked Spartans. He's averaging 6.5 points per game.
Michigan State opens Big Ten play Sunday, then faces Oakland University on Dec. 14 in Detroit.
The Spartans have already had to deal with one major injury. Senior guard Joshua Langford had a setback in his return from a foot injury and hasn't played this season. Izzo said recently he is not counting on Langford coming back.