The Blues did not reveal how long they expect the 32-year-old to be sidelined, other than to say Krug is expected to be reevaluated Oct. 1.
St. Louis opens the season Oct. 12 at Dallas. Krug's injury is a pre-training camp blow to the organization that's attempting to return to the playoffs quickly after a rare down season and trading away Stanley Cup-winning forwards Vladimir Tarasenko and Ryan O'Reilly.
Krug is going into his fourth season with the Blues since signing a seven-year, $45.5 million contract in 2020. The former Boston Bruins defenseman has 112 points in 185 regular-season and playoff games with St. Louis.
If Krug misses time early in the season, it would create a spot for one of the Blues' young defensemen, either Tyler Tucker or Scott Perunovich, to get a regular role on a blue line that includes veterans Colton Parayko, Justin Faulk, Marco Scandella, Nick Leddy and Robert Bortuzzo.