A day after Steelers linebacker James Harrison told reporters that his surgically repaired knee was “A-OK” for Sunday’s game against the Denver Broncos, the 2008 defensive player of the year was limited in practice Wednesday and his status for Sunday is now uncertain.

Missing Harrison would be a big blow to the Steelers, but his absence would not be the biggest problem the Pittsburgh defense would have to worry about.

That honor, or lack thereof, would go to free safety Ryan Clark, who will miss Sunday’s game due to the sickle cell trait he has.

Clark nearly died after a game in 2007 caused him to lose 35 pounds, along with his spleen and gall bladder in the high altitude of Denver.

Clark’s absence was one of the main reasons why the Steelers made Tim Tebow look like Joe Montana in Pittsburgh’s 29-23 loss to the Broncos in the Wild Card round last season. Tebow threw for 316 yards while cornerback Ike Taylor played possible the worst game of his career, along with safety Troy Polamalu looking like a shell of himself all afternoon.

With Clark on the field, it allows Polamalu to play closer to the line of scrimmage and wreak havoc on opposing offenses, which helps take some of the pressure off the cornerbacks on the outside.

Without him, it forces Polamalu to stay deep in coverage, giving the quarterback more time to look the field over and find open receivers, something Tebow did with ease against the Steelers in January.

Veteran backup Ryan Mundy will start in place of Clark, and while he is more than capable of stepping in and starting, he doesn’t bring the same element Clark brings to the defense.

Clark is the quarterback of the secondary, an underrated, Pro Bowl caliber player, and without him the Steelers, as they did against Denver in the playoffs, look lost.

Perhaps the Steelers should consider themselves lucky to catch Peyton Manning and his new offense early on before they can truly get on track. However, if the Steelers can’t find a way to play effective pass defense against Manning on Sunday, they may be in for a long night.