He missed practice Friday and was questionable because of a sore knee.
Harrison tweeted it would be a game-time decision whether he plays against the Broncos, but he is expected to miss the game.
Linebacker Larry Foote (ankle) is expected to play.
Linebacker Stevenson Sylvester is also expected to miss the game with a knee injury and is about a week away from returning.
Also Friday, running back Rashard Mendenhall was listed as doubtful as he continues his recovery from a torn ACL. Backup Isaac Redman said his ankle feels much better and indicated that he and Jonathan Dwyer would split carries against the Broncos.
DeCastro sustained a left knee injury during the team's third preseason game.
Harrison was not removed from the physically unable to perform list until last week and made it through practices Monday and Wednesday in a limited capacity. Traditionally under coach Mike Tomlin, players who do not practice Friday do not play Sunday.
Harrison acknowledged his knee, which underwent minor surgery last month, was experiencing pain and discomfort and that some soft tissue had yet to heal.
Information from The Associated Press was used in this report.