The defenseman hasn't played since he was boarded by Blues forward Oskar Sundqvist late in the first period of Game 2. Grzelcyk has been in the NHL concussion protocol, and coach Bruce Cassidy said after the morning skate that he hasn't been medically cleared. Grzelcyk, who has seven points (three goals, four assists) in 19 games this postseason, skated a second straight day with the Bruins at practice Saturday but did so in a non-contact jersey.
Sundqvist was suspended one game for the hit.
The Bruins need to win to extend the best-of-7 series and prevent the Blues from winning the Stanley Cup for the first time.
RELATED: [Complete Stanley Cup Final coverage]
Defenseman Zdeno Chara and forwards Noel Acciari and Marcus Johansson were full participants at practice after incidents in the past two games.
Chara sustained a facial injury during Game 4. He played 16:42 wearing a protective shield in Game 5, when the Bruins used 11 forwards and seven defensemen. Boston will dress 12 forwards and six defensemen for Game 6, with Karson Kuhlman as the 12th forward and defenseman Steven Kampfer a healthy scratch. Forward David Backes will be scratched for the second straight game.
Acciari left Game 5 with 9:24 remaining in the third period for concussion protocol after hitting his head on the ice.
Johansson was hit in the head by Blues forward Ivan Barbashev in the first period of Game 5 but played the rest of the game. Barbashev was suspended for Game 6 for that hit.
Johansson worked with the first power-play unit at practice Saturday, switching spots with forward Jake DeBrusk. The Bruins are 0-for-5 in the past two games since they went 4-for-4 in Game 3.
"He's a little more of a distributor than Jake," Cassidy said of Johansson. "Jake's more of a net-front presence. I think they both recover pucks well. Marcus is good on entries; he's used to having it through the neutral zone. We had a few issues there the other day, so hopefully he gives us another option."
Boston was 0-for-3 in a 2-1, Game 5 loss.
"It's hard to say [what should change]," said Bruins forward Charlie Coyle, who plays on the second unit. "It's not one thing. The power play's been great (the Bruins lead the playoffs at 33.3 percent). Sometimes you get a good bounce, sometimes you don't. Sometimes you think you should score and you don't and it feels a lot worse than it is. We can't let our power play hamper our 5-on-5 game. Get a power play? Take advantage, play simple, do what you can and move the puck around."
