The team made the announcement Wednesday morning. Head coach Mike Pettine said the day before that both McCown and Manziel would prepare to play, but McCown did not practice Tuesday night.
"On the short week we realized that Josh just wasn't going to be able to get to the point where he could go," Pettine said Wednesday. "Johnny has worked hard to prepare himself. He is excited about his opportunity and we are all looking forward to the challenge we face in a good Bengals football team on Thursday night."
McCown, who has started seven games for the Browns this season, injured his ribs in a loss to the Arizona Cardinals last Sunday. He also has been dealing with an injury to his throwing shoulder.
Manziel, in his second year in the NFL, played near the end of the game last week in his first action since leading the Browns to a 28-14 win in the second week of the season over the Tennessee Titans. In four games this season, Manziel has completed 28 of 50 passes for 393 yards, three touchdowns and an interception, with five sacks.
Manziel made his first career start against the Bengals last season in a 30-0 defeat, in which he was intercepted twice, sacked three times and compiled just 80 yards of passing.
"He's a different player,'' Pettine said Tuesday before practice. "I think we saw that this year when he had to go into the Jets game. I thought he did some good things, made some mistakes, put the ball on the ground then had a week of preparation to get ready to Tennessee and went out and executed a gameplan. I just think we've all seen it from going back to the amount of work in the spring to training camp.''
Pettine said the major difference from last year is Manziel's patience in the pocket.
"When the rush became live, I think that's where you saw that he made his biggest improvement with his demeanor in the pocket,'' Pettine said Tuesday. "That it wasn't as frenetic, that his feet were calm. I just think he has a really good understanding of what we're doing, understands the plays, what's designed for what. All the intricate details, he has a much better grasp of and it's shown when he's been out there."
Manziel met with NFL investigators last week regarding his role in the Oct. 12 incident with his girlfriend in Avon, Ohio. Manziel's girlfriend initially told police that Manziel hit her a "couple times" and pushed her face against a car window.