Zdeno Chara scored with 13 seconds left in regulation after David Krejci tied it late, lifting the Bruins to a 3-2 win over Pittsburgh on Saturday night in a game that saw Orpik taken from the ice on a stretcher.
Midway into the opening period, Thronton was given a match penalty for grabbing Orpik from behind, pulling him to the ice and punching him when he was down. Orpik appeared to be dazed by a punch, but may have also hit his head on the ice.
The Penguins said on their Twitter site that Orpik returned from Massachusetts General Hospital and will be traveling with the team back to Pittsburgh, according to Penguins coach Dan Bylsma.
''Obviously I made a mistake,'' Thornton said. ''I'm aware of it. I've been told that they're having a hearing and it's hard for me to say much more other than that's not my intention. I feel awful. I feel sick.''
Thronton, known as the Bruins' enforcer, knows he went over the line that comes with his role.
''It's always my job, I guess, to defend my teammates, but I've prided myself for a long time to stay within the lines,'' he said, speaking very quietly in the dressing room. ''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.''
Reilly Smith also scored for the Bruins, who haven't lost in regulation at home since late October (10-0-2).
Tuukka Rask stopped 28 shots for Boston.
Bruins coach Claude Julien said both teams should take some blame for physical play that went over the line at times.
''The only thing I'm going to say is it's not just one way here,'' he said. ''There's a lot of blame to go around. We can all learn from it.''
Marc-Andre Fleury stopped 18 shots for Pittsburgh.
Penguins captain Sidney Crosby felt an early hit by Orpik led to emotions running high.
''I think Orps' hit on (Loui) Eriksson probably led to what happened with Thornton and all the things prior to that,'' he said. ''So, yeah, I think that's a big reason why.''
Julien said Eriksson left with a concussion and will not travel to Toronto for Sunday's game.
Krejci scored with 1:29 to play, beating Fleury with a wrister from the slot. Chara unloaded the winner from near the middle of the faceoff circles, cutting in from the right point.
''It was playoff-kind of hockey,'' Chara said. ''We were behind that whole game and fighting for that tying goal. Eventually we got it and carried that momentum through the rest of the game, and we were able to score the goal.''
The Penguins led 2-1 after one period on the goals by Kunitz and Neal.
Neal was surprised by Thornton's actions.
''I'm not going to comment on that, obviously never like to see anything like that,'' he said. ''Thought the confrontation was over after you know he asked him to fight a few times and kept going after him.''
Kunitz made it 1-0 at 6:33 when his centering shot/pass from the bottom of the left circle caromed into the net by Rask off the body of Chara, who was at the edge of the crease.
Boston had tied it 1-all when Smith unloaded a slap shot over the left shoulder of Fleury before Neal beat Rask with a wrister between the pads at 13:13.
Orpik's injury came after he was in a scrum with a few Penguins players with Boston's Gregory Campbell in the Pittsburgh defensive end of the ice.
Boston's Brad Marchand was injured in front of the Bruins' bench near center ice before Thornton came charging down and grabbed Orpik. Players on the ice immediately called for medical help and he was surrounded before being taken off the ice to a polite applause from the crowd.
Neal was assessed a kneeing penalty for his hit on Marchand.
Some of the bad blood between the teams came from the game's opening minute when Orpik hit Eriksson in the head with his shoulder as the winger started to head up ice. Eriksson was dazed as he slowly made his way to the bench and was helped down the runway to the dressing room. He didn't return.
Early in the third, Smith missed a wide-open side of the net when his shot went over the crossbar. Chara hit the right post with 2 1/2 minutes to play.
Boston swept the Penguins last spring to advance to the Stanley Cup finals, which it lost in six games to the Chicago Blackhawks.
In the teams' previous meeting, Crosby scored with less than a second left in regulation before Torey Krug scored the winner for the Bruins in overtime.
NOTES: Julien said earlier in the day that D Johnny Boychuk would miss ''three to seven days'' and the club is planning on bringing him on a four-game road trip that begins Sunday at Toronto. Boychuk was taken off the ice on a stretcher Thursday in Boston's loss in Montreal, but Julien said ''we feared the worst and I think we got as good of news as we could get.'' Boychuk had an MRI Friday that found no fractures. ... Penguins C Evgeni Malkin missed his second straight game with an undisclosed lower body injury. Malkin was the NHL's leading scorer in November. ... Kunitz headed to the locker room for a while in the second after taking a stick in the chin from Marchand, who was given a double minor. ... It was the third and final regular season meeting of the season. Boston won two.