Pittsburgh’s Ryan Clark understands Coach Mike Tomlin’s decision not to play him in Sunday’s playoff game in Denver as a precaution against the possibility his sickle-cell trait could become aggravated during exertion at high altitude. (Tony Tribble/Associated Press)

When everything fell into place Sunday night and the NFL’s playoff matchups were set, the Pittsburgh Steelers gathered their thoughts about what it would mean to play in Denver against the Broncos. There would be the Tim Tebow factor, and the questions about how to deal with the specter of the Broncos’ running quarterback. There were concerns about the Steelers themselves, because running back Rashard Mendenhall blew out his knee and is done for the year, and quarterback Ben Roethlisberger suffered another tweak to his balky ankle in the regular season finale against Cleveland.

Those are issues for which the Steelers could prepare. When Ryan Clark figured out the opponent and the site, his eyes welled up.

“If I was going to be in any more danger than any other player on the field,” Clark said, “I knew there was a chance that I wouldn’t play.”

The last time Clark played in Denver, in October 2007, it cost him two organs, because he has sickle cell trait, which causes a blood reaction when he exerts himself at high altitude. Denver, of course, sits a mile above sea level. Clark remained in Colorado following the game, and within a month, he had his spleen removed. His gallbladder followed after that. At one point, he said he was “scared, real scared,” enough that he wondered about greater consequences. He lost 30 pounds.

Four years later — and six long years since the Washington Redskins decided to sign safety Adam Archuleta and let Clark walk — Clark faced the same situation. He was torn. He is, by now, an anchor for a Steelers’ defense ranked first in the NFL in both yards and points allowed. He is Pittsburgh’s leading tackler. He felt as if he were healthy enough to play, that the risks had been analyzed and assessed.

Yet he also knew Coach Mike Tomlin and his care for his players. And he knew that, for all the analysis, he hadn’t heard one thing.

“They couldn’t tell me 100 percent: Nothing’s going to happen to you,” Clark, 32, said. “You’re going to play and you’re going to be fine.”

After Clark missed the remainder of the 2007 season following his operations, he and his doctors began a series of tests to determine whether he’d be able to play in Denver, should the situation arise again. On a preseason trip there in 2010, they ran him through drills prior to the game, though he didn’t actually play. They put together an elaborate plan for any Denver game that involved keeping Clark hydrated, giving him a variety of intravenous fluids during the game, using oxygen both on the plane and during the game, and taking more IVs at halftime.

“I was comfortable with it,” Clark said. “. . . This is the postseason. If we lose this one, this is it.”

But Monday, Tomlin spoke with Clark. It was, Clark said, a “super-short, one-sided” conversation. Tomlin’s answer: No.

“We’ll keep it in perspective,” Tomlin told reporters Tuesday. “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.”

The message, though, was more personal to Clark. Tomlin said if either of his two sons had Clark’s condition, he wouldn’t allow him to play. Therefore, he wouldn’t permit Clark to, either.

“When a man tells you his reasoning for not letting you play is because if that was his son, he wouldn’t want him out there, it shows that he cares about you more than just as a football player, that you’re not just an asset to the football team,” Clark said. “He understands you have a family and people that depend on you.”

Clark’s Washington past — the 2004 and 2005 seasons — is now long ago, in football terms. “No one cares about me there!” he said Wednesday. But he still carries a bit of the Redskins with him to practice every day. His uniform number on Sundays is 25. His uniform number in practice is 21, an homage to his former partner at safety, the late Sean Taylor.

“If you truly loved somebody,” Clark said, “you never let it go.”

Wednesday, as the Steelers began preparations for the Broncos, Clark wore No. 21 out to practice. His role was reduced to mimicking Denver safety Brian Dawkins.

“He was rolling around, flying around,” said Ryan Mundy, the fourth-year player who will replace Clark on Sunday. “That’s my big brother.”

It is, this week, the only time for Clark to have fun, the only role he can play. He said he will serve as an intermediary between the players, Tomlin and defensive coordinator Dick LeBeau on Sunday. He is studying film. But if the Steelers don’t beat the Broncos, his season is over.

“To not be able to play with your teammates is a tough situation,” Clark said. “But also to have to wonder after every play’s over if you’re going to be all right, that’s also a stressful situation.”