Sundqvist will return to the lineup Monday after serving his suspension in Game 3, which Boston won 7-2 on Saturday.
There are two days off before Game 5 at Boston on Thursday, which gives Grzelcyk more time to recover. Game 6 would be at St. Louis on Sunday.
John Moore filled in for Grzelcyk in Game 3 and played 13:09 on the third defense pair with Connor Clifton.
Grzelcyk has seven points (three goals, four assists) and is averaging 16:22 of ice time in 19 Stanley Cup Playoff games.
Though Grzelcyk remains in Boston, forward Chris Wagner, who is sidelined with a right arm/hand injury, is with the Bruins in St. Louis but has yet to resume skating. Wagner hasn't played since blocking a shot from Carolina Hurricanes defenseman Justin Faulk in Game 3 of the Eastern Conference Final on May 14.
Cassidy didn't sound optimistic about Wagner being able to play during the Final.
"He's working out with the team. He's not on the ice yet," Cassidy said. "We'll give you a better update once he starts practicing. They're just not that close until they're on the ice."