Osborne's decision was not SOLELY for the benefit of Lawrence Phillips. Yes, Ahman was amazing and no, it turns out we didn't really need Lawrence to beat Florida. But that game was supposed to be more competitive, and the smart move is to get your Heisman candidate running back on the field (purely from a football perspective).
Putting Lawrence Phillips back on the team and on the field to "help him" implies there weren't other ways to help him, including counseling, incarceration, etc. Football was not the only way to help him. It just so happened that football also greatly benefited Tom Osborne.
It also happened to be the only option that Osborne had direct control over.
And apparently counseling (I assume) and incarceration (obviously) didn't help either.