Njoku suffered the injury on Monday Night Football in the Browns' victory over the New York Jets. The third-year pro also suffered a concussion in the contest.
The Browns haven't revealed the exact diagnosis for the injury; early reports suggested Njoku broke his wrist, but those rumors haven't been confirmed.
Njoku posted four receptions for 37 yards and one touchdown in his first two outings of the season. He had 56 catches for 639 yards and four scores in 2018.
In a corresponding move, Cleveland claimed safety Juston Burris off waivers from the Oakland Raiders.