This is a surprising move. Bill has never been known for adding assistants to his staff in the middle of a season. He brought back Josh because he knew O'Brien was leaving for Penn State and he had a prime opportunity to acquire one of the top offensive minds on the market.
Daboll is an offensive coach. He worked as WR coach during his first stint here, before fleeing to the Jets to become QB coach. He worked ass OC in Miami and KC too.
This could give rise to speculation that someone on the staff has told Bill they're leaving. Top guess would be McDaniels, but he has said publicly many times that he is satisfied being in New England for the moment. I tend to believe him, as I think he'd rather build his credentials over a 2-3 year period rather than jump from a 1 and done.
This will be interesting.