Orpik, 33, was placed on injured reserve on Monday.
He left the ice on a stretcher just past the halfway mark of the first period in Saturday’s 3-2 loss to the Bruins.
Orpik was involved in a scrum and pulled to the ice by Thornton, who then began punching the defenceman who lay on his back. Orpik was later released from Massachusetts General Hospital, where he was alert and conscious.
Thornton, who was assessed a game misconduct, will have an in-person hearing with the NHL’s department of player safety at not-yet determined date. He could receive a suspension that exceeds five games.
After Saturday’s game, Thornton told reporters he felt horrible for his actions.
""It's hard for me to talk about right now. I can't say I'm sorry enough. I'm sure I'll be criticized, but it's true," he said. "I felt awful. It wasn't my intention.
I know Brooksie. I've gotten to know him the last seven years here. I skated with him during the lockout. I texted him a couple of times. It's not what I wanted to see or anyone to see."
Prior to Thornton's actions, Boston's Brad Marchand was kneed in the head by James Neal, who was handed a five-game suspension by the NHL on Monday. Earlier in the game, Orpik nailed Bruins forward Loui Eriksson, forcing him to leave the contest.
Orpik has appeared in all 31 of Pittsburgh’s games this season, posting a goal, seven points and plus-1 rating.
He joins top-four Penguins defencemen Paul Martin (leg) and Rob Scuderi (ankle) in the infirmary.
The Penguins have recalled blue-liner Brian Dumoulin from Wilkes-Barre/Scranton of the American Hockey League to fill Orpik's roster spot.
The 22-year-old has one goal and six points in 22 games this season for the AHL Penguins, and could make his NHL debut on Monday against Columbus.