Thornton went after Orpik during a stoppage in play a week ago, slew-footing him to the ice and punching him twice in the head. Orpik suffered a concussion and was taken off the ice on a stretcher and transported to the hospital.
Earlier, Orpik hit Boston winger Loui Eriksson, knocking him out of the game with a concussion. NHL vice president of player safety Brendan Shanahan pointed to the two-minute roughing penalty Thornton received for trying to get Orpik to fight as a sign of premeditation.
"This cannot be described as a hockey play that went bad, nor do we consider this a spontaneous reaction to an incident that just occurred," Shanahan said in the video announcing the longest regular-season suspension during his tenure as the league's disciplinarian. "It is our view that this was an act of retribution for an incident that occurred earlier in the game, the result of this action by Thornton was a serious injury to Orpik."
Orpik has not played since the game, which was Dec. 7 at TD Garden in Boston, and is on injured reserve. He is still experiencing concussion-like symptoms and took part in a light skating session for the first time Friday.
Speaking to reporters at Joe Louis Arena in Detroit before his team's game against the Red Wings, Penguins coach Dan Bylsma said he was not waiting for the NHL's ruling to be satisfied with what happened.
"He's a pretty honest hockey player who made a mistake," Bylsma said of Thornton. "[Shanahan] made a ruling, I think, that says volumes about getting that kind of play out of the game."
Thornton had already served three games of this ban before the Bruins' 6-2 loss to the Vancouver Canucks Saturday night. He is eligible to return Jan. 11 when the Bruins face the San Jose Sharks.
Boston coach Claude Julien wouldn't comment on the suspension.
"I said what I had to say when it happened," he said. "The league made a decision and I move on with the team."
Prior to the game, Julien had praised Thornton's leadership.
"We take a lot of pride in making sure that everybody knows they're important on our team, and Shawn's been a real good player for us," he said. "Him missing in our lineup, we've lost a little bit of his leadership. We've lost his presence as far as what his line brings to us."
Most of the Bruins politely declined comment.
"If that is how the league is approaching it, that's their decision," said forward Brad Marchand. "We will live with it."
The suspension comes after Thornton's in-person hearing with the department of player safety Friday in New York.
Thornton's agent, Anton Thun, said in an email to The Canadian Press that no decision had been made whether to appeal the suspension. It is the only one longer than 10 games that Shanahan has given since taking this position in June of 2011.
"I am aware of today's ruling by the NHL department of player safety," Thornton said in a statement released by the team. "I will be consulting with the Bruins, my representation and the NHLPA about next steps, and will be in a position to address the matter publicly after speaking with those parties."
Thornton forfeits US$84,615.45 in salary as a result of the suspension, his first disciplinary action during his NHL career.
"We respect the process including the ability to attend and present our case in person," Bruins general manager Peter Chiarelli said in a statement. "At this time, we will decline comment until the process is complete and Shawn has exhausted all rights available to him."
Penguins forward James Neal was suspended for five games earlier this week for kneeing Brad Marchand in the head during the same game.
The 36-year-old from Oshawa, Ont., has three goals and 41 penalty minutes in 27 games this season.