Bruins head coach Bruce Cassidy announced the update and noted Grzelcyk "looks like he'll go in" the lineup in the place of Connor Clifton.
The 25-year-old Boston native suffered a concussion in Game 2 on a hit by Blues forward Oskar Sundqvist, who received a one-game suspension for the play that caused the injury.
Grzelcyk missed the last four games, including the Bruins' 5-1 road win in Game 6 on Sunday to force a seventh and deciding game for the NHL championship.
The Boston University product tallied three goals and 15 assists for 18 points in 66 games during the 2018-19 regular season, his second full year in the NHL. He had seven points in 19 playoff appearances before suffering the injury.
Although he hadn't been cleared for full contact, he's been practicing with the Bruins since last Wednesday with hope he'd receive the green light before the end of the Final.
"The toughest thing is just not being out there with the team," Grzelcyk told reporters last week. "Having to sit and watch is obviously tough when you get to this stage. I'm just happy to be back around the guys. I'm feeling a lot more like myself and hopefully I get cleared to play."
His expected return should bolster the already-favored Bruins' chances of raising their seventh Stanley Cup and first since 2011. They are listed as -175 favorites (bet $175 to win $100) in Wednesday's game, per Vegas Insider.
Faceoff for Game 7 is scheduled for 8 p.m. ET on NBC at TD Garden in Boston.