Even with Darren Sproles likely to take on a bigger role, Tomlinson's return strikes me as a boost for his fantasy value.
Tomlinson could have landed somewhere else where he'd be expected to carry the full load, but that was unlikely, and Sproles' role might not increase all that significantly. The biggest issue is Tomlinson's health and the play of the Chargers' offense line which was unexpectedly poor last season.
I'd argue Tomlinson right now is still a top-15 back.