When quarterbacks get hurt for also-rans like the Eagles, it's an opportunity to see what the younger guys can do. When the injured QBs are stars such as Ben Roethlisberger, and they play for contenders, it's a whole other situation — and not often a pleasant one.
If Michael Vick's concussion sidelines him Sunday at Washington, that's unfortunate for the Philadelphia starter, of course. But in the next-man-in NFL, third-round draft pick Nick Foles also will get a chance to show if he is the Eagles' future at the critical position.
The scenario is entirely different in Chicago, San Francisco and Pittsburgh, where one man's injury could potentially wreak a team's playoff aspirations.
The Eagles (3-6) can afford to take a look at Foles. Barring a stunning turnaround, they won't come close to making the playoffs, and they've given no indication their dysfunction offensively, defensively and organizationally is going to disappear.
Vick has been a turnover machine, as much the cause of Philadelphia's flop as anyone. If Andy Reid's job as coach is on the line, as most everyone looking from the outside seems to believe, Vick doesn't seem capable of saving it. So why not go with Foles?
"I have a lot of confidence in him. That's why we brought him here," Reid said.
"He's confident in his ability. If it comes down to where he has that opportunity, I think he'd be excited and (with) his personality type, it would be more of a positive than it would be a negative. I think he's wired that way."
And because the Eagles almost certainly won't chase a playoff berth down to the wire, the switch from Vick to Foles wouldn't be too painful in competitive.
The Bears, Niners and, especially, the Steelers, have more at stake.
Chicago already has learned a bitter lesson, going from 7-3 in 2011 to 8-8 and out of the postseason after Jay Cutler went down and Caleb Hanie failed as his replacement. That collapse contributed mightily to general manager Jerry Angelo's ouster, and the new regime immediately addressed the void by bringing in Jason Campbell, who has a history as a starter and far better credentials than Hanie.
With Green Bay breathing down the Bears' necks, they need for Campbell to come through Monday night at Candlestick Park, and perhaps beyond that, if Cutler can't go.
"Going all the way back we felt like we needed to improve our backup quarterback position in the offseason and feel real good about convincing Jason to come here to be Jay's backup," coach Lovie Smith said. "He has been very good in limited time we have given him ... going against a very good defense we have here.
"We feel he is a starting quarterback in the NFL and we feel very good having him here."
Campbell needs to perform at the level of a starter, something he has been for much of his eight-season career. He was playing relatively well last year when he broke his collarbone and Oakland traded for Carson Palmer.
His potential counterpart Monday night, Colin Kaepernick, barely has a pro resume. In his second NFL season, Kaepernick has completed 16 of 26 passes in 2012 and rushed for 177 yards and three touchdowns. Last year, with Alex Smith taking nearly every snap, Kaepernick barely got onto the field.
Smith certainly could be the starter against Chicago, but if he isn't, the 49ers (6-2-1) can't afford to give Kaepernick any learning curve. Their tie at home against St. Louis last weekend brought them closer to Seattle (6-4) in the NFC West, and they still have visits to New Orleans, New England and Seattle ahead.
"Whoever's behind center, we'll be ready to go," left tackle Joe Staley said.
So will the Steelers, who always come to play regardless of their injury list. The defense has solidified even without Troy Polamalu, and the running game broke out even though Rashard Mendenhall and Jonathan Dwyer have been sidelined at times.
Now comes the biggest challenge, and the timing couldn't have been worse for Pittsburgh. Roethlisberger has shoulder and rib injuries, and division leader/archrival Baltimore is up next, and then again in two more weeks.
We'll hear all kinds of brave words out of the Steel City about Byron Leftwich stepping in, and having a veteran with a solid knowledge of the offense and a bazooka for an arm is a positive for sure. But remember how often Roethlisberger has been the difference between beating out the Ravens in the AFC North — or eliminating them in the playoffs.
"Let's be honest, I'm not going to run around, make 2-3 guys miss, roll all the way to the left and find Mike Wallace in the back of the end zone," Leftwich said. "I'm not capable of doing that. But what I can do is get the ball in the right people's hands and just be myself."
He also needs the running backs to be themselves — at least the versions that have lifted Pittsburgh to sixth in league rushing. And the wideouts to continue their strong play that has carried the Steelers to the top in yards gained passing.
And for the defense to be, well, a Steel Curtain.
Even then, all of Pittsburgh will be rooting for a quick return by Roethlisberger, no matter how well Leftwich performs.