As NFL Media Insider Ian Rapoport pointed out, there was confidence McCown would be cleared this week, but concussions are an unpredictable lot.
Manziel has been dealing with elbow tendentious, which has kept him out of some portions of practice this week. Austin Davis is likely to be the backup Sunday, in the event Manziel gets hurt.
Manziel replaced McCown early in last week's Browns loss to the New York Jets. The second-year signal-caller went 13-of-24 passing for 182 yards, a touchdown and an interception for a 75.3 rating. He added 5 carries, 35 rush yards, but lost two fumbles. Manziel led the Browns in rushing yards Week 1.
Manziel's 182 yards passing last week eclipsed his tossing total from the entire 2014 season (175 yards).
As Marc Sessler broke down this week in the Around The NFL Film Room, it was an up-and-down performance for Manziel.
While Cleveland wanted to turn back to their hand-picked veteran starter, McCown's concussion paves the way for further evaluation of Manziel. The franchise needs to find out if their first-round pick will sink or swim. Tossing him into deep waters is the only way to find out.
As for Mariota, he will now face Heisman Trophy winners in his first two NFL starts -- first Jameis Winston, now Manziel. Mariota will become the first Heisman-winning quarterback in NFL history to face two former Heisman-winning QBs in back-to-back games at any point, let alone his first two NFL games.