The Browns are concerned that he tore his patellar tendon, a source told ESPN's Adam Schefter. Conklin will have additional tests on the knee Monday.
Conklin went down on the Browns' second offensive possession. After being helped to the sideline by the Browns' training staff and his teammates, he was carted to the locker room.
Conklin had just returned from a dislocated elbow that he suffered in Week 8. He was an All-Pro selection in his first season with Cleveland in 2020.
"You hate that for anybody, especially Jack, who's been fighting to come back after the dislocated elbow," Browns quarterback Baker Mayfield said after the game. "It's just one of those freak deals. He's one of our guys, and it's just a really, really unfortunate thing."