Jackson injured his pectoral muscle during training camp. He tried to return to practice this week but didn't feel comfortable. Jackson suffered a different pectoral injury last year that also knocked him out for the season. The Browns signed receiver Sam Aiken with the open roster spot.
Cleveland also downgraded receiver Brian Robiskie (hamstring) and defensive lineman Shaun Rogers (ankle, hip) to "out" for Sunday's game against the Baltimore Ravens. Receiver Josh Cribbs also was downgraded to "questionable" with an ankle injury.