New England Patriots quarterback Tom Brady hopes Rob Gronkowski's knee injury is not too serious and admits it would be difficult to replace the tight end.

Gronkowski was carted from the field during Sunday's game at Denver Broncos after taking a hit from safety Darian Stewart to his right knee.

Tight end Gronkowski was limited to seven appearances in the 2013 season after seriously injuring ligaments in the same joint.

The Patriots went on to lose 30-24 in overtime as their perfect record this season was ended, and Brady - who had thrown to Gronkowski for a touchdown in the first quarter - is hoping the diagnosis for his team-mate is not too severe.

"He has been through a lot," Brady said. "He is a tough guy. Hopefully it's not too serious and our fingers are crossed.

"I hate to see it, but it [hitting low] really is the only the way defenders can hit now. I bet if you asked the players they would really rather go high than low.

"I don't think it's dirty I just think that is how football is played now.

"He is the best tight end in football. It is so hard to see these guys get hurt this way.

"I always have so much respect for guys that play this game because you risk a lot to play. I think a lot of times that gets overlooked because you sacrifice a lot and guys sacrifice their bodies.

"It is hard to see your friends get taken down like that. It is part of this sport and we understand that, but I also have a lot of respect for guys that take the field."