The Browns made the series of roster moves after Sunday's 38-6 blowout loss to the Baltimore Ravens. To replace Seibert, who missed an extra point and one field goal in the defeat, the team signed Cody Parkey off their practice squad to the 53-man roster.
Seibert -- a fifth-round pick out of Oklahoma last year -- misfired on an extra point after the Browns' touchdown in the first quarter Sunday. Near the end of the second frame, he missed a 41-yard field goal wide right.
Njoku was put on IR with a knee injury, according to the team. Njoku, who scored the Browns' lone touchdown against the Ravens on Sunday, will now miss at least three games.
Njoku suffered a sprained MCL in Sunday's game and is expected to miss three weeks, according to ESPN.
The Browns (0-1) host the AFC North rival Cincinnati Bengals in Week 2.