Schwartz, who had four assists in seven games, was taking part in a warmup drill. He shot a puck and reached to get the rebound when he lost an edge and crashed into the net where goalie Brian Elliott was practicing.
Schwartz was transported to the Washington University Orthopedic Center, where he had surgery.
He will miss at least 41 games.
"I didn't think we anticipated the length of it," Blues coach Ken Hitchcock said before the Blues played the New York Islanders on Saturday (8 p.m. ET; MSG+ 2, FS-MW). "Today is more of a scramble day. We'll just see what we got and then we'll really buckle down here over the weekend and prepare for how we're going to manufacture our lineup, so right now, it's kind of play this game as a sudden-death game, probably play a little shorter bench than we wanted to, and find out what looks like it sorts itself out."