Sharks center Joe Thornton has shocked everyone before when it came to returning sooner than expected from a serious knee injury. This time, though, no one knows when he might be able play again. Thornton is out indefinitely after he underwent arthroscopic surgery Thursday to address an issue with his injured right medial collateral ligament, the Sharks announced just prior to the start of the team’s game with the New York Rangers. The latest update did not give an exact timetable for when Thornton might resume skating or playing, although Sharks general manager Doug Wilson said Wednesday that Thornton’s injury would cause him miss “several weeks.” Thornton had an MRI on his right knee Wednesday, and underwent an additional medical evaluation Thursday afternoon. Thornton’s MCL injury occurred in the late stages of Tuesday’s game against the Winnipeg Jets, as he collided with Sharks forward Mikkel Boedker near center ice with roughly 44 seconds left in the third period. Whether the latest news on Thornton heightens the Sharks’ need to add a player between now and the Feb. 26 NHL trade deadline remains to be seen. Wilson didn’t rule it out Wednesday, saying, “Probably not. But don’t hold me to that. You never know what becomes available as you go forward.” The Sharks were already in the market for a fourth line center. But with Thornton, still the team’s emotional leader at age 38, out for an indefinite period, Wilson will almost assuredly explore options for how to bolster his club for the playoff drive, depending on how it performs in the coming weeks.