It's OK, Osi. In a broader sense, we're all day-to-day. Getty Images

[Ed's note: For a first-person account of injury from Cardinals receiver Larry Fitzgerald, please go here.]

Tom Brady spent the week of Super Bowl XLII pretending. When he walked around Arizona, he didn't cringe, sigh, grind his teeth or give any other indication that his right ankle was killing him. But the night before the game, one of Brady's closest friends whispered to me that the QB's injury was "worse than you can imagine." He said it particularly affected Brady's mobility and his ability to throw deep. Sure enough, in the loss to the Giants, Brady was sacked five times, hit countless others and threw several errant passes.

Brady still won't admit that the ankle affected him. And that's one of the reasons his teammates love him. Inside NFL locker rooms, nothing is more valued than a player's ability to play through pain. Just ask Chargers QB Philip Rivers, who made his legend by suiting up for the AFC championship game just six days after surgery to repair a torn meniscus. "He's the most courageous man I have ever seen," San Diego defensive end Luis Castillo said in the locker room following his team's loss to the Pats. "For him to play—nobody will ever comprehend what that means."

That's because success in the league is built upon control. Drills, playbooks and film work are all meant to eliminate the curse of the unpredictable. But there's no making sense of the one element that can't be controlled: injuries. So when a player overcomes them—when he decides a tweaked hamstring or shredded ligament won't dictate his or his team's destiny—it can inspire awe. And when he doesn't, well, it can be debilitating. Not just for the team. The path from injury to admiration is fraught with pitfalls—some of which can derail a career.

That's why every player (and fan) needs to know the unwritten rules below. They provide the NFL blueprint for playing in pain.

Midway through last season, 49ers QB Alex Smith tried to accelerate his return from his first major NFL injury, a separated right shoulder. When teammates asked how his rehab was going, he responded like he thought a tough guy should: "I'm fine."

Wrong answer. Smith's shoulder wasn't close to being healed. But not wanting to let anyone down, he kept the extent of the pain to himself and rushed back onto the field. Then his forearm seized up in practices, balls missed their targets in games and Niners coach Mike Nolan publicly questioned his signal-caller's confidence. After three dreadful performances and having lost his teammates, Smith benched himself and had season-ending shoulder surgery.

Smith, the first overall pick of the 2005 draft, didn't know any better, but he should have put the timetable for his return squarely on the medical staff. When teammates asked, "Are you ready?" he should have said, "I'm trying, but the trainers think I'm a week away." That way Smith would have gotten credit for busting his ass but had a viable out clause if he came up short. Now Smith is the Niners' No. 2 QB. But if put in the same situation, he now knows better. "I'd have to be honest with myself and ask, If I go out there, am I giving us the best chance to win?" he says. "I'd sit down with the trainers and coach and figure out a plan. I'd do the unselfish thing."

Even if it's not the macho thing.

None of the Pats heard Brady whine about his injured ankle, and none of the Chargers heard Rivers give himself credit for toughing it out. The two QBs didn't try to elicit sympathy because they knew their teammates were playing injured as well. "Hurt is such a broad term," says Giants center Shaun O'Hara. "Everyone is hurt by midseason. Ultimately, it just matters if you can block out the pain."

In the locker room, only a few injuries are widely accepted as severe enough to miss multiple games. These include broken legs and collarbones; torn ACLs, MCLs and medial menisci ("the unhappy triad"); severed rotator cuffs; ruptured Achilles tendons; herniated disks; and spine and neck injuries.

Players won't raise their eyebrows if someone sits out with a concussion, high ankle sprain, pulled groin, pulled hamstring or cracked ribs, so long as only one game is missed. But they expect everyone to play through everything else, including shoulder separations, wounded digits (this goes even for quarterbacks), hip pointers, cracked ribs, stingers, turf toe and, in some cases, torn and sprained knee ligaments. "An injury that'd keep out one guy won't keep out someone else," says Cardinals QB Kurt Warner, who's suffered a slew of thumb injuries during his 11-year career. "Is it fair to weigh the two against each other? No, but that's how it's done. There's always something in the back of everyone's mind: Okay, is he really tough enough?"

Judgments about a player's toughness vary not only by injury but by status as well. Ex-Seahawk Shaun Alexander sat out three weeks with a broken wrist last year, while Indy's Marvin Harrison took nearly three months for his sprained knee; neither star was dogged by teammates. But a rookie or fringe player isn't afforded such leeway. Says Seahawks defensive end Patrick Kerney, "You have to earn that right."

Of course, the rules change when the stakes are high. LaDainian Tomlinson pulled himself from last season's AFC title game after playing in two series with a sprained left MCL. That move invited criticism from the likes of Jim Brown and Deion Sanders that Tomlinson is still trying to shake. No one seemed to care that he truly believed the Chargers had a better shot at winning by playing his healthy backups in his place. Nor did anyone care that by staying in the game, he risked permanent damage to his knee. Still, says Tomlinson: "I wouldn't have done anything differently."

After all, he'd be taking hits either way.

Some GMs have what's called an 80/20 rule: If a player is on the field at 80%, don't add 20% to his production. Translation: Suits don't give the benefit of the doubt when players push themselves to play too early. Every NFLer who tries to leverage his willingness to ignore pain into a bigger contract learns that bottom-line lesson. "I tell my players not to come back too soon," says one agent. "There's such peer pressure to do so. But when it's time to negotiate, if the player's numbers are down, the club doesn't care."

"Once you step onto the field," says Dolphins quarterback Josh McCown, "you're saying you're 100%. No matter what, there are no excuses."

Even if you deserve one.