I've been saying for weeks that we're only two deep at the SF spot, and one of those two guys is going to be 35 (and always dealing with nagging injuries) and the other is coming off heart surgery. The deep-backup options are Courtney Lee, who's barely played SF in the NBA and gives up a lot of size, and Joseph, who's a rookie second rounder who may not even make the team.
Lee may be fine at SF against some opponents, but certainly not against Miami or any other team playing more of a "power" 3. Lee in a smallball lineup is something you want to be able to do situationally (and something I think Doc will do a fair amount), but really shouldn't be something that one injury forces you to do regardless of the opposition.
Delfino would be perfect, but even short of him I'd feel a lot more comfortable with one more legit SF on the roster.
Pietrus would be great, too, but it sounds like he'll go to Europe before he signs another vet-min deal.
Guys like those two aren't going to be in any rush to sign contracts for what the C's can offer, so I suspect we won't see any more signings until we get pretty close to camp. I do think there's one more player to be added, and that Joseph and Christmas are fighting for one roster spot.
And yeah, they'll be looking for Jeff Green to play a fair amount of PF (probably half his minutes); you aren't paying him a starter's salary to play 15MPG behind Pierce. He probably could have gone elsewhere and started; he's back in Boston because he's getting well-paid and because Doc's going to carve out a large role for him.