An NFL season not only requires survival of the fittest, but being the most fortunate.

When injuries strike, sometimes it doesn’t matter how talented a team’s roster is. A few big- time hurts will severely damage their championship chances.

Yes, the Packers provided the perfect argument against that last year when they went all the way despite losing 16 players to season- ending injuries, including six starters. But that was an anomaly, something just about everyone in Green Bay recognized as they were eagerly welcoming back the likes of Jermichael Finley and Ryan Grant, finally healthy for the 2011 season.

More prevalent is seeing teams go into the tank when key players are sidelined. Prime example? Peyton Manning and the winless Colts. The big question marks now are in Chicago, Houston, Baltimore and Oakland. All four teams believe they can emulate the 2010 Packers.

Bears quarterback Jay Cutler, heavily criticized for sitting out in the second half of the NFC title game last January with a knee injury, finished off Sunday’s win over San Diego despite a broken thumb on his right hand. His throwing hand.

Caleb Hanie performed well in staging a mini-comeback in that playoff loss to the Packers, but he hasn’t thrown a pass this season and remains an unknown quantity as Chicago ( 7- 3) tries to stay in front in the NFC wildcard race. True, Cutler can be unpredictable, even erratic, but he seems to be mastering coordinator Mike Martz’s offense and cutting down on the game-turning mistakes and poor decisions.

“We’re going to miss a great player for a period of time,” coach Lovie Smith said.

Yet, the injury could be survivable for Chicago if the defense, special teams and running back Matt Forte keep playing the way they have. And, no more injuries would be helpful.

“Caleb’s been around here a long time and the team feels comfortable with him leading us,” Smith said. “There’s a lot of good things happening. Don’t feel sorry for us or anything like that. We have a lot of things in place.”

So, it would appear, does Houston. The Texans ( 7- 3) never have made the playoffs, but have a two-game lead in the weak AFC South, have won four straight, have a terrific running game and an emerging defense, and will be getting back star receiver Andre Johnson this week.

They also are without their starting quarterback, Matt Schaub, out for weeks with a right foot injury. Along with the season-ending injury to star linebacker Mario Williams, that could spell doom for Houston.

Or not.

“ The guys are confident. It’s the ‘next guy up’ kind of attitude,” said linebacker Brian Cushing, who knows the next guy behind center is previously underachieving Matt Leinart. “We know we’re going to step up, we know we’re going to play better football. We lost a hell of a leader, a hell of a football player. But at the same time, other guys are going to have to play. Everyone’s play is going to rise. The sky’s the limit for us. We’ve got six games left and we’re going to try to take care of them.”

The worries in Baltimore about making defensive stands while Ray Lewis is on the sideline waving towels and growling at the opposition were relieved a bit with the three interceptions, two sacks and numerous big plays against Cincinnati. Still, the Ravens’ offense is just spotty enough that they’ll need the defense to be staunch in the final six games.

Luckily for Baltimore, leaders and stars abound on that unit, from Ed Reed to Haloti Ngata to Terrell Suggs.

“Like I told the guys, there are 53 leaders on this team, and the guys know it,” Reed said. “ The communication wasn’t there all the time with Ray not being there. We missed it. But guys made plays.”