Let's get this out of the way right up front: The Chargers are better off with a healthy LaDainian Tomlinson in the lineup. Even if he's started the descent stage of his career, Tomlinson's still capable of carrying them for a drive, a half or a game when his body allows it. His body isn't going to allow him to do much, if anything, in Pittsburgh on Sunday.
That shouldn't kill the Chargers' chances of pulling off an upset, however. You needn't look any further than November for reasons why San Diego shouldn't fret. The Steelers eked out an 11-10 win at Heinz Field that Sunday, and the Chargers only got 57 yards on 18 carries from Tomlinson. Darren Sproles should be able to pick up three yards a carry against the Steeler defense, and may have more big play ability in his tank than Tomlinson at this point in time.
It's not just about replacing Tomlinson in the running game, however. The Chargers will have the benefit of providing the Steelers with a harder offense to read than they had in November. Without Tomlinson, their approach is going to be a mystery until the first drive. The Steelers can't just sit back and prepare for the usual rushing attack and load up to stop it, and they can't just assume Philip Rivers will pass the ball all over because Sproles is more than capable of being a factor.
The other reason why Tomlinson's absence isn't a huge factor is because the Steelers were the better team even with him in the lineup. Losing Tomlinson isn't bringing the favorite closer to the underdog, it's working in the opposite direction. That means that the expected result is an even more likely one.
None of that should bother the Chargers. They knocked off the favored Colts in Indianapolis in this round last year with an injured Tomlinson, Rivers and Antonio Gates playing at less than 100%. There may be a greater disparity between the teams this time around, but the Chargers have no reason to throw up their hands and give up because Tomlinson won't be playing.