The New England Patriots star tight end, who missed the first six games recovering from off-season forearm and back surgeries, was injured just before the midpoint of the third quarter Sunday and driven off the field on a cart.
Gronkowski suffered a suspected torn anterior cruciate ligament in his right knee and is expected to undergo an MRI on Monday. Coach Bill Belichick said he was taken to a hospital for evaluation.
The Patriots beat the Cleveland Browns 27-26 on Tom Brady's two touchdown passes in the last 61 seconds.
"We've sustained some pretty big injuries this year with really important, critical players," Brady. "I think we all feel sorry for Rob, but I don't think anyone feels sorry for the Patriots."
Gronkowski raised his right arm to the crowd as he left on the cart with team physician Dr. Thomas Gill sitting next to him.
Gronkowski was hurt on a hit by Browns safety T.J. Ward after a 21-yard reception with the Browns leading 12-0. Ward was among many players from both teams who went up to Gronkowski before he left the field.
"My intention is never to hurt anyone. That's not what this game is about. That's not how I play," Ward said. "I hate to see guys go down with any type of injury. I just wanted him to know, whether he accepted it or not, it wasn't intentional to injure him. But we have to play this game. We have to play it the way that they force us to, and unfortunately, it incurred an injury for him."
Ward said if he hit Gronkowski high, there was a chance he might be fined.
"It's not a decision where I'm going to go low on every play," he said, "but, actually, it has kind of turned into that in those situations, in those split-second decisions where you have a guy running down the field who's 280 (pounds), and I'm 200, and the fact that if you do even hit him in his chest and he decides to hunch down or flinch or lower his head, then I could have the chance of being fined."
"It's kind of being caught between a rock and a hard place," Ward said. "It's a decision you have to make, but you have to follow the rules at the same time."
He said he had been fined three times and "I don't like playing for free."
Asked if he had an issue with Ward hitting Gronkowski low, Belichick said, "That's a question you should take up with the league."
The Patriots (10-3) have won three straight games and hold the No. 2 spot in the AFC despite numerous injuries.
Gronkowski, wide receiver Danny Amendola, running back Shane Vereen and defensive players Vince Wilfork, Jerod Mayo and Tommy Kelly all have missed an extended amount of time.
In the six games before Sunday, Gronkowski led NFL tight ends with 37 catches and 560 yards receiving. He had two catches for 32 yards Sunday.
Now he's out again.
"As a team, we want him to play," Patriots cornerback Devin McCourty said, but "you just hate to see that for him. A guy that's battled back through his arm and his back (injuries) and been very productive for us this year, it stinks to see him get carted off the field like that."
AP NFL website: http://www.pro32.ap.org