Johnson was evaluated for a concussion on the sidelines and was cleared to return for the game, according to NFL Around the League and the game broadcast.
The Bengals already lost Robert Geathers to an elbow injury earlier in the game, so the depth at defensive end has been wearing thin. Johnson had 11.5 sacks in 2012 and Geathers had three sacks a year ago. That's quite a bit of production to lose in one game, even if Cincinnati's tandem of defensive tackles is one of the best in the league.
Johnson's injury appears to be minor. He is one of the team's top defensive players. The Bengals haven't given Ben Roethlisberger much time to throw on Monday night, and Johnson has been one of the main reasons why. Upon re-entering the game, Johnson pressured Roethlisberger, so he hasn't shown any ill effects from the head injury.