ESPN.com's Bill Williamson first reported on Sunday that Clark would sit out.
Clark nearly died following a game in Denver two years ago because of a rare blood disorder that is aggravated by playing in high altitude.
Clark had his spleen and gall bladder removed during separate operations and he lost 30 pounds after the Oct. 21, 2007, game in Invesco Field that ended his season. Many of Clark's teammates told him he should skip this game.
About 2½ hours before kickoff, Clark went onto the field in a long-sleeve black shirt and baggy black shorts with white tube socks and ran several times from one sideline to the other, walking back each time, resting for a minute and then repeating.
Steelers coach Mike Tomlin told the team's Web site he didn't want to risk Clark's health or burden his teammates and family members with worry.
"I think it was the appropriate thing to do in this situation. I didn't want to be worried about his physical well-being, and I know his teammates would be, plus his wife and family. By not allowing him to play, I think we all can be singularly focused on what we came to Denver to do, which is win a football game," Tomlin told Steelers.com.
Tomlin said Clark took the news well.
"He's fine. Ryan is a professional. He's a team guy, and I think he respected the decision," Tomlin told the Web site.
Information from The Associated Press was used in this report.