During the preseason Fab was terrible on offense - he was downright hideous.
BUT he was playing very good Pick and Roll defense, he was playing EXCEPTIONAL single-coverage defense, and he was blocking shots like a boss.
On a per minute basis his rebounding and assist numbers were also pretty good.
The guy has skills and talents, he just has a major lack in confidence and needs time to adjust to the NBA game.
To me Fab looks like his confidence is on the floor. He cannot earn a single minute of game time - he's not even respected enough to earn a spot as a bench warmer on a team that desperately lacks frontcourt defense. For him, imagine how much that would kill your confidence? Will being send down to play in some D-league team help that confidence? Will playing for a different team who runs a different system help his adjustment to the Celtics philosophies?
If he's on the Celtics bench, even if he doesn't play a minute, I think it will benefit him more. He gets to watch his teammates play, he gets to listen in on huddles, he gets to involve himself in practices. Maybe in the occasional blowout he can go in and play 3 or 4 minutes. It will at least make him feel like he belongs and like the guys want him there. I think for somebody in his position that would be a huge thing for him.
Plus like I said, how much worse than Collins can he be? Collins does nothing but pick up fouls and allow opponents to score.
Fab + Sully could actually be a surprisingly competent combination. Sully has great hands, is an excellent rebounder, and has a good scoring touch inside. Fab is long, quick and agile and an excellent defensive player. He could offer quality help defense when Sully's man gets past him (in much the same way as KG does) and wouldn't really NEED to be involved offensively.