Clark has a rare blood condition that nearly caused him to die after his last game played in Denver in 2007. Clark was hospitalized in Denver when complications from sickle-cell trait prevented oxygen from flowing into his spleen. He later needed operations to remove his spleen and gall bladder, and the 205-pound Clark lost more than 30 pounds before he finally began feeling better more than two months later.
Clark received medical clearance to play Monday night, however, the source said the Steelers decided it wasn't worth the risk. He is listed as questionable on the team's injury report.
Clark will be on the field during pregame warm-ups, but will only play if there is a rash of injuries in the secondary before the game and it becomes an extreme emergency, the source said.
The Steelers are not expected to officially announce the decision until the inactive players are announced about 90 minutes prior to Monday's game.
While other players with sickle-cell trait can play in Denver without problems, Clark's condition was worsened because he is among the estimated 1 percent of those affected by the condition who react poorly to exerting themselves in high altitude. Denver is 5,280 feet above sea level.
Bill Williamson covers the NFL for ESPN.com. The Associated Press contributed to this story.