I think if Brooks is in the Celtics, Doc will let him play some meaningful minutes, at least in the first few games.
People were talking about getting Nick Young and Jamal Crawford, Brooks isn't far behind those guys but is much younger and cheaper. If he can play for the Celtics then the 2nd unit will have a lot more fire power.
Again I don't understand what do we gain by exchanging Brooks with Johnson, who's getting the least minutes at the moment. It'll probably take two seasons for Johnson to grow to a reliable bench player, while Brooks is ready now.