According to ESPN’s John Clayton New England Patriots Rob Gronkowski was carted off the field with a lower leg injury.
Rob Gronkowski being taken off the field on a cart because of lower leg injury
— John Clayton (@ClaytonESPN) December 8, 2013 
Source: Josh Alper, NBC Sports Pro Football Talk 
The tight end took a shot to the right knee from Browns safety T.J. Ward while trying to catch a pass from Tom Brady.
It was a scary looking hit in real time as Gronkowski’s leg buckled and he immediately went down with his hands on his helmet. He was helped onto a cart and taken back to the locker room a short time later.