Steelers Sideline Safety Clark Because of Blood Trait Justin K. Aller/Getty Images The Steelers decided against risking the health of safety Ryan Clark and sidelined him for Sunday's wild-card game at Denver. PITTSBURGH - Ryan Clark peeked in the window a few moments before Coach Mike Tomlin's news conference began Tuesday, a smile creasing his face as he was about to become the most unusual entrant on the playoff injury report. Tomlin had met earlier with Clark, the Steelers" safety, and told him that he would not be allowed to play on Sunday, when the Steelers face the Broncos in an A.F.C. wild-card game in Denver. Clark, the Steelers" leader with 100 tackles this season, has a sickle cell trait that makes him susceptible to organ damage if he exerts himself at high altitude - and the Broncos" stadium is a mile above sea level. Clark knew about the trait, but was unaware of the severity of the risk, when the Steelers played at Denver in 2007. The altitude caused a reaction in Clark's blood that attacked his spleen. Over a few weeks, in separate operations, doctors removed Clark's spleen and gall bladder. At one point, when a doctor told him that they would pray together as Clark lay in a hospital bed, Clark feared that he was going to die. He lost more than 30 pounds and did not play again that season. Clark recovered, but he has not played in the Steelers" two games at Denver since, one in the regular season and one in the preseason. Before the preseason game in 2010, the Steelers had Clark run and work out in the high altitude, but at an intensity level far lower than he would experience in a game, to test how his body responded. Clark had no trouble, but Tomlin, who consulted with team doctors, decided to play it safe and said it was an easy decision to hold Clark out of Sunday's game. "It is a big game for us, but it is a game," Tomlin said. We'll keep it in perspective. If he's in any more increased danger than the other 21 men on the field, then we're going to err on the side of caution. Clark had said he wanted to play, but his public lobbying was strikingly muted for a gregarious and outspoken player, an indication that he, too, was worried about what could go wrong if he played again at that altitude. Tomlin said his conversation with Clark was not much of one at all. He called Clark in, told him he was out, and Clark said O.K. Steelers players had Tuesday off, but Clark, writing on his Twitter feed, sounded relieved that Tomlin and the team had made the decision for him. "Well I guess we all know now," Clark wrote. I will not be playing. Glad that it's out now. So no more questions to ask. Thanks for your concern! Clark is not the only loss for the Steelers this week. Running back Rashard Mendenhall tore a knee ligament in the regular-season finale and is out for the playoffs. Isaac Redman will be the primary back in his place. Clark will make the trip with the Steelers, and Tomlin said he expected him to help the sideline communicate with players during the game. Last year at the Super Bowl, Steelers safety Troy Polamalu singled out Clark's play as critical to Polamalu's success. Polamalu, the defensive player of the year in 2010, said Clark's ability to cover deep downfield freed Polamalu to freelance and pursue plays. But Clark has also played closer to the line of scrimmage, a role that Polamalu will probably take on this week against the Broncos, while Ryan Mundy, a sixth-round pick in 2008 who has two career starts and has mostly filled in for Polamalu, will drop deep in the defensive backfield. The Steelers could also use Will Allen to fill in. Whoever plays, Tomlin said, the Steelers expect "Ryan Clark-type football." Tomlin said of Mundy: "He's starting capable. We're comfortable with what he's capable of doing. We don't anticipate us missing Ryan Clark to hinder us schematically. The Steelers" decision to minimize Clark's risk probably does little to increase their own against the Broncos. The struggles quarterback Tim Tebow has had with his throwing and his accuracy are well documented, and few opponents considered the Broncos" passing game a significant threat, even during their six-game winning streak. Tomlin said he saw Tebow as a player who "just wins." He went on to delineate the intangibles that have defined Tebow's ascendance: his ability to play well in significant moments, to get his teammates to play hard, to lead with a natural charisma. When asked if Tebow could be an effective passer, Tomlin responded, "Certainly." But in the Broncos" last three regular-season games - all losses - Tebow's improvement arc flattened. He completed 30 of 73 passes (41.1 percent) for 439 yards, 1 touchdown and 4 interceptions, figures that seem to reduce the importance of a deep safety. The Steelers, as they do in every game, will focus first on stopping the run, where the Broncos, with Tebow augmenting an attack led by Willis McGahee, are ranked first in the league in rushing yards and attempts. The Steelers, though, have the league's top-ranked defense. "We're not going to take anything for granted," Tomlin said. Especially with one of their top defenders on the sideline.