Decker, the team's leading receiver, was helped off the field and went straight to the locker room.
The second-year receiver hauled in a pass over the middle, but was immediately tackled by Harrison, who went low to bring Decker down on a play first ruled a catch and then overturned on replay.
Harrison has a history of hits to the head. He was suspended last month because of his helmet-to-helmet hit on Cleveland quarterback Colt McCoy, who suffered a concussion.
Already banged up on the offensive line, the Steelers had left tackle Max Starks leave with a right knee injury and his return is doubtful. They're already without All-Pro center Maurkice Pouncey, who missed the game with an ankle injury.
Pittsburgh defensive linemen Brett Keisel (groin) and Casey Hampton (left knee) left with injuries and their return was questionable.