Reader went down with less than nine minutes in the third quarter of the 27-3 loss after he tackled Baltimore's Gus Edwards following a 3-yard gain. Reader was carted off the field and was visibly frustrated as he went back to the locker room, according to ESPN's Jamison Hensley.
Defensive lineman Sam Hubbard also was ruled out of Sunday's game with an elbow injury.
Reader was a key addition during the Bengals' spending spree this offseason. Reader signed a four-year deal worth $53 million, which is believed to be the richest contract given to a free agent in franchise history.
In his first four games of the season, Reader tallied 18 tackles, with one of those coming behind the line of scrimmage.