The forward was injured with 1:07 left in the first period of a 2-1 shootout win at the San Jose Sharks on Friday. He collided with teammate Kyle Clifford in front of the Sharks goal.
St. Louis played at San Jose on Saturday.
"He's an all-around player for us, lots of different situations for us, penalty kill, power play, checking, energy, and really our team feeds off him," St. Louis coach Craig Berube said before the game. "It's a tough loss for sure. We're going to miss him, but we've got guys that are going to come in and fill in, and everybody's just got to pick up the slack. That's all you can do."
Sundqvist will require surgery when the Blues return to St. Louis and will be reevaluated at training camp prior to the 2021-22 NHL season.
Forwards Jacob de la Rose, Dakota Joshua and Nathan Walker played Saturday in place of Sundqvist, Tyler Bozak (upper body) and Zach Sanford (COVID-19 protocol).
"Someone's going to come on and play," Berube said. "It's unfortunate. It's tough news for 'Sunny,' and us obviously."
The Blues also are without forwards Robert Thomas (thumb), Ivan Barbashev (ankle) and Mackenzie MacEachern (upper body), and defensemen Carl Gunnarsson (knee) and Colton Parayko (upper body).
Sundqvist scored nine points (four goals, five assists) in 28 games this season. The 26-year-old has scored 72 points (32 goals, 40 assists) in 229 NHL games in six seasons.
NHL.com independent correspondent Louie Korac contributed to this report
