I don't think Boston needs a wing, per se, but if the right guy comes along sure. They absolutely need a big guy though.
This is the way I generally see the roster playing out
Playoff Rotation (tighter/more minutes)
PG - Smart 36, White 12
SG - Brown 24, Brogdon 24
SF - Tatum 34, Brown 14
PF - Horford 12, Grant 30, Tatum 6
C - Rob 30, Horford 18
Grant is the problem and that is a bigger issue in the regular season when the minutes are lessened. Grant shouldn't be playing that many minutes and I don't want Tatum playing many more than 6 at PF. There is also a huge issue if either Rob or Al goes down. That rotation completely cuts Pritchard out and White doesn't play all that much. Adding a wing just makes that worse. Again you don't pass on talent, like a TJ Warren at a vet minimum contract, but the focus shouldn't be on a wing.
The team needs at least 1 big man and frankly probably 2. Maybe one of them is Bryant, but he is so injury prone he makes Rob look like an ironman. Plus, he also isn't that good and can't play PF at all.
I'd take a look at Dwight Howard. And I know he can't play PF, but I think he is the right kind of player for the team i.e. a guy that will rebound, block shots, and can handle a lob. Team needs some toughness down low. He is the right guy.