The 28-year-old defenseman sustained an upper-body injury during a 3-2 overtime win against the Colorado Avalanche on Friday and left the game with 9:41 remaining in the third period. He didn't play in a 6-1 loss to the Arizona Coyotes on Saturday.
Blues general manager Doug Armstrong said Pietrangelo will be re-evaluated around Christmas.
"He's talking to the doctors now," Armstrong said. "There's the likelihood that he'll have some surgery, yes."
Pietrangelo has 11 points (four goals, seven assists) and averages a Blues high 24:37 of ice time in 24 games.
"He brings a lot to us," Blues defenseman Colton Parayko said. "He'll play all situations on the ice. Obviously our leader back there. It's one of those things where you've just got to be excited if you're a defenseman in the lineup to come to the game and be prepared and take on a role, a bigger role that you're already in, just embrace it and take it head on and make sure you do the best of your ability. Just do your job and help the team as much as possible."
The Blues (9-13-3) play the Edmonton Oilers at Enterprise Center on Wednesday (8 p.m. ET; NBCSN, SN, TVAS).
Also going on IR was forward Robby Fabbri (separated shoulder). Armstrong said Fabbri also will be re-evaluated around Christmas.
Defenseman Carl Gunnarsson (upper body) was placed on IR retroactive to Nov. 17.
Fabbri, 22, was injured during the first period against the Coyotes. He has four points (two goals, two assists) in 15 games after missing the first month of the season with groin, hip and back injuries. He missed most of the past two seasons after tearing the ACL in his left knee twice, causing him to miss 122 of the past 137 regular-season games.
"I don't think it's a surgical candidate," Armstrong said. "I'm actually sure it's not or else we'd be a lot longer than Christmas."
Forward Alexander Steen is day to day with a concussion sustained against the Avalanche.
"We all know what situation we're in," Blues center Brayden Schenn said. "Obviously key players being out, but at the same time, we're not using this as an excuse. It's an opportunity for myself and other guys to step up and try and win hockey games."
Forward Jaden Schwartz (hand) and Gunnarsson have each missed the past eight games, but Armstrong said they're day to day and getting closer.
St. Louis recalled forwards Jordan Kyrou and Jordan Nolan, and defenseman Chris Butler from San Antonio of the American Hockey League.
"It gives other guys opportunities that are going to be presented to them now," Blues coach Craig Berube said. "Everybody's got to step their game up. Injuries happen in this League. It's part of it all. Every team goes through it."
