The NHL's department of player safety announced the suspension Thursday after a hearing with Sundqvist, who was assessed only a two-minute minor penalty for boarding for the hit that knocked Grzelcyk out of Game 2 and possibly longer.
Sundqvist and Grzelcyk are each out for Game 3 on Saturday. Grzelcyk didn't travel with the Bruins, and coach Bruce Cassidy said the 25-year-old was in concussion protocol and considered day-to-day.
While some Bruins players were reluctant to weigh in on the hit, former Blues captain David Backes was critical in the aftermath of Wednesday's Game 2.
"It's from behind, elevated, into his head, into the glass," Backes said. "If that's a two-minute penalty, I think there's going to be a shortage of defensemen in this series by the end of it. That's in somebody else's hands. That's something I think if I'm making that hit, I'm probably watching from the bleachers for a few."
"He's a big part of our team," captain Alex Pietrangelo said about Sundqvist. "He plays big minutes night in and night out and in every situation."
Blues coach Craig Berube didn't want to elaborate on Thomas' status but said upon landing that Dunn was close to returning after missing five games for a puck to the face. He gave no indication whether Zach Sanford would step in to the vacant forward spot or if St. Louis might instead dress seven defensemen.
After Robby Fabbri stepped in for Thomas in the Blues' Game 2 overtime victory and defensemen who had sat out are now contributing, Berube isn't worried about his team's depth in this situation.
"It's been a big factor, for sure, not only in the playoffs but throughout the last month or two in the season," Berube said. "Our depth is tremendous on the back end and up front. We use everybody. It's been very important."