Blues general manager Doug Armstrong said Friday the forward would probably be sidelined until early December.
Toropchenko (6-foot-6, 222 pounds) scored two goals in 28 regular-season games as a rookie this season and had two assists in 12 Stanley Cup Playoff games.
"It is a tough blow, but it is one of those ones where he could have potentially played with it and then if he got injured again, then you're looking at a 4-5 month surgery," Armstrong said. "We just thought it was prudent to get it taken care of now so A) it's strong and when he comes back, he doesn't have to worry about it."
The Blues (49-22-11) finished third in the Central Division. They lost in six games to the Colorado Avalanche in the best-of-7 Western Conference Second Round.
"[Toropchenko] went from a call-up player, to a regular player, to a player (coach) Craig [Berube] would use higher up in the lineup," Armstrong said. "Size, skates, physical, great team guy. It's strange to say for a guy that's [played] less than 40 or 50 games or whatever it is, but we're going to miss him in October and November and we can't wait for him to get back."
The 2022-23 regular season will begin Oct. 11 in North America. The Nashville Predators and San Jose Sharks will play two regular-season games in the 2022 NHL Global Series at O2 Arena in Prague, Czech Republic, on Oct. 7 and 8.
NHL.com independent correspondent Lou Korac contributed to this report
