My prediction as to how the Chargers running back situation plays out.
1) LaDainian Tomlinson takes a pay cut to stay with the Chargers. He knows that he is limited and he is pretty smart in the business side of football. If he doesn't take a pay cut, he gets released, and there is no way any other team signs him for anything close to his current contract.
He will be getting a reduced salary no matter what, so he will take the high road and reduce that salary for the Chargers. He saw what happened with Brett Favre in New York and Emmit Smith in Arizona. He will still be VERY marketable as a Charger (get ready for more Campbell's Chunky Soup and Vizio commercials).
2) The Chargers will draft a running back this year, but I do not know which round. Chargers General Manager, AJ Smith, does a great job finding quality starters farther down in the draft and RB is one position that you can find speed and vision at various points and have them step in and contribute right away (think Steve Slaton and Matt Forte).
The Chargers will transition to more of a two-back system (think Maurice Jones-Drew and Fred Taylor in Jacksonville) and this will allow Tomlinson to improve his average yards per rush and be more effective later in the season, but he may barely get over 1000 yards and 10 TDs.
3) Darren Sproles will be offered money as a kick/punt returner and third down, situational running back. AJ Smith will make it very clear that he will never get 300+ carries with the Chargers. He will also try to convince him that he will be less effective that way, but as a returner/screen guy, he is one of the most exciting players in the league.
Then the ball will be in Sproles' court. If he goes the Napoleon Complex route and wants a full time No. 1 running back spot , some other team will offer him the money (especially after Michael Turner's success). It is very possible his agent could convince him he is capable of 300+ carries at 4.0 yards per carry. I can only hope Sproles realizes his value and decides to stay with the team that gave him a chance coming out of college.