There's already been some discussion in the blogosphere about which O'Neal should start in Perk's absence, so I figured I'd chime in with my opinion.
I was under the impression that the Celtics, when signing JO, promised him to the starting job until Perk returns. I believe in keeping your word, so if the Celtics made a promise, they need to keep it.
If they DIDN'T, however, then Shaq should start. JO is slated to be the first big off the bench once Perk returns (at least, that's what I've been hearing), so I think it would be wise to get him acclimated to that situation as soon as possible and have him back up Shaq. This team -- with newcomers Shaq, JO, Wafer, Bradley, probably Harangody and possibly another SF -- will have plenty of acclimating to do without adding more to it.
Though I advocate the C's keeping their word if they indeed made a promise, I also think it would be fine for them to go to JO and say, "Hey, we know we made a promise and we're willing to keep it if you hold us to it, but will you reconsider taking that starting role, since we'd like you to get used to the backup role you'll be taking on eventually anyway?"
I realize that this could cause a problem for Shaq -- being a starter for awhile, then being relegated to no better than the third big off the bench after Perk returns (Perk starting, JO first off the bench) -- but I think it's in the best long-term interests of the second unit to have JO and the rest of the second-stringers fully used to each other.
And there might be a unique opportunity here for Shaq: being the sole member of the 1A unit. Someone (I think it was Chris Forsberg) wrote that Shaq might be better off playing with the starters because they (particularly KG) could do the best job of compensating for Shaq's pick-and-roll-defense deficiencies. Well, Shaq could start, as I've said, until Perk returns, and after that he could be a sort of segue between the starters and second unit: Let's say Perk is gassed or in foul trouble and Doc wants to pull him, but he wants to leave the rest of the starters in for a bit longer, he can put in Shaq with the rest of the first unit, then later bring on the B team, including JO and Baby.
So that's what I think. Chime in. I know you will.