Demps (6-foot-2, 195), a native of San Antonio, originally injured his shoulder during a summer basketball game. He originally planned to have surgery, but later opted against it.
Demps played scarcely during four games of his freshman season, averaging just 2.3 points per game.
"The injury really limited Tre and it was in his best interest to get it taken care of at this time," said NU head coach Bill Carmody in a statement. "By getting the surgery done now, it should allow him to return to full strength by late spring."
In Demps' absence, Northwestern will now be forced to count on freshman David Sobolewski to carry a larger load, while hoping Alex Marcotullio and Jershon Cobb can return to their regular load of playing time.
Northwestern currently stands at 7-1 on the season, and prepare for a stretch of three games in four days, beginning on Thursday night with Texas Southern.