I'm going to assume Murphy is going to sign (from the reports out there) so I'll add him to the mix. We would have Shaq, Jermaine, Murphy, Kristic, Baby, and KG all capable of playing center.
Shaq I would think most of us would agree should be the starter but he can only go 20 to 25 minutes a night tops.
After that things get a little more blurry. Doc and Danny have said they have been searching for someone who could fill a similar role as Posey and go small in crunch time moments. You could see a lineup of KG at center, Green at PF, Pierce at SF, Allen at SG, and Rondo at PG when it matters most. That is one option.
Baby bodies up well against big centers but I would think he wouldn't be playing center at all if everyone is healthy.
Jermaine seems logical to me (again if fully healthy) as the first center off the bench because of his defense, shot blocking, ability to score a little and hit the outside shot.
As the third string between Murphy (again I'm assuming we sign him) and Kristic I think I would choose Murphy as he is a better rebounder, a better scorer, and ability to hit the 3 (career 39% from 3).
Kristic was a starting center on a very good team. He could very well be in street clothes if everyone is healthy. It would be a little ironic if he sits alot come playoff time.
How would you divide the minutes between PF and C?
KG at 32 MPG maybe some minutes at C in a small lineup with Green at PF.
Shaq 20 MPG
Baby 16 MPG
Jermaine 15 MPG
Green 8 MPG per game at PF rest of his minutes as Paul's backup.
Murphy 5 MPG
Kristic sitting, Powe too if we got him.
It could very well be much different than that depending on Doc's thinking.
The good news from the "Encouraging words from Ainge" thread is that it looks like we will have both Shaq and Jermaine. The byproduct of that is that it would appear none of our big guys would have to play very long minutes.