The problem is that they won't ever have enough money to sign Adrian no matter what happens. San Diego has a very apathetic fan base and even a nice playoff run wouldn't bring much of a spike in attendance. If they trade him in the off-season, they can get more for him than they could if they waited until the trade deadline.
His ceiling in Boston would be as a 4th outfielder. If you can get a solid reliever for him (wouldn't happen, but maybe you get someone on a day when they drank too much cough syrup) you do it without thinking twice.