Pietrangelo was injured late in the second period of Game 1 after he was bumped from behind by Kings rookie right winger Dwight King. Pietrangelo fell awkwardly into the boards, cutting his chin.
King was given a two-minute boarding penalty, and Pietrangelo did not return to the game, which the Kings won 3-1.
Pietrangelo, who had 12 goals and 39 assists in the regular season while leading the Blues in ice time, sat out Game 2 with the undisclosed injury. The Kings won that game 5-2 for a 2-0 series lead.
"It's hard to believe that one player makes that big a difference but obviously, in our game, he did," Blues coach Ken Hitchcock said.
Meanwhile, the Kings are close to getting injured fourth-line forward Kyle Clifford back from a concussion, coach Darryl Sutter confirmed. Clifford was injured in Game 1 of the first-round playoff series against the Vancouver Canucks.
Clifford practiced Wednesday and skated with the team Thursday morning at Staples Center. Sutter said Clifford would be a game-time decision.