Assuming Springer isn't going to be part of the rotation, I think the position the Celtics could use some depth would be one more big guard/quick forward. Furkan Korkmaz, Danuel House, Delon Wright... Someone like that.
On the other hand, no buyout candidate is likely to factor into the rotation much so my choice would be Gallo. I just think, from a vibes standpoint, bringing him back and giving him a chance to win a championship as a Celtic after not being able to play at all last season would be great. And he's a little more insurance at the 4/5.