I would expect Mozgov to be a backup center here behind Garnett.
I think there are two different roles he could have within that backup center designation. A medium sized bench role and a miniature sized bench role.
(1) If Boston landed a starting caliber big man (Josh Smith please!) then Mozgov's minutes would be severely limited behind that new big man, Garnett and Sullinger. So Mozgov would probably only play about 5-10 minutes a game and only get 2-3pts and 2rpg.
(2) If Boston fails to get such a big man, then Sully likely starts alongside KG with Bass taking over the backup PF duties. Sully would double up and play some minutes at center alongside Bass but would still leave an increased amount of minutes for Mozgov. Probably 12-15mpg when roster is fully healthy that would go up to around 20mpg over the course of the season due to injuries. I think 4-5pts and 4-5rpg would be expected in that role.