Clark has a sickle cell trait, and the high altitude in Denver negatively affects his condition. He played a regular-season game in Denver during the 2007 season and became violently ill after the high altitude caused his blood to sickle, eventually losing his spleen and gall bladder due to a deprivation of oxygen.
Clark then sat out following two trips the Steelers made to Denver (a regular-season game in 2009 and a preseason game last year).
"It was really kind of a easy decision for us," Tomlin said. "If he's in any more increased danger than any of the other 21 men on the field during the course of a football game, then of course we were going to err on the side of caution.
"It's a big game for us, but it is a game," Tomlin added. "We'll keep it in that perspective."
Tomlin said he met with Clark to tell him about the decision Monday. Ryan Mundy will start in Clark's place.
"Not much of a conversation," Tomlin said. "I talked to the medical experts. I called Ryan in and told him he wasn't playing. He said OK."
"On this treatment table thinking about the game. Thinking of how blessed I am to still be here & how Id play if I (Mike) T would let me #crazy," Clark tweeted, before adding: "Mike Tomlin is without a doubt one of the best most unique head coaches ever. I'd run through a wall for him. Great coach, better man!"
Tomlin also confirmed that running back Rashard Mendenhall suffered a torn anterior cruciate ligament in his right knee against the Cleveland Browns and would miss the rest of the season. Mendenhall was placed on injured reserve Tuesday, and no date has been set for his surgery.
Tomlin said quarterback Ben Roethlisberger (ankle) is doing OK but might be limited early in the week, and guard Doug Legursky (shoulder), linebacker LaMarr Woodley (hamstring) and fullback David Johnson (elbow) should all be fine. Tomlin considers running back Mewelde Moore (knee) and cornerback Cortez Allen (right shoulder) questionable for the game.