Thornton experienced swelling in the knee and flew back to San Jose to meet with doctors Saturday while the rest of the team headed to the East Coast for a four-game road trip. Thornton could return Oct. 14 at the earliest.
Thornton, 39, was limited to 47 games last season with knee issues before he underwent surgery in January, but Sharks coach Pete DeBoer downplayed the injury this time.
"He had a little bit of swelling after the last game, and rather than continue on the road and risk anything with it, we're going to take the safe route and get him home and get it looked at," DeBoer said, according to The Mercury News.
Thornton has an assist in two games this season after scoring 13 goals and adding 23 assists last season.
The Sharks recalled center Dylan Gambrell from the AHL.