How many game are we getting from O'Neal? How much of a dropoff in production from the big three will we have? How will Rondo look compared to his start last year? How will the offense look with a decent shooter (Bass) taking Baby's minutes? How will Green do this year? There are a number of questions with the team. I doubt they'll all all go south. The starting unit could easily be better than last year, the bench (we still need another player or two) could be as well.
These questions are all valid ones. The biggest 2 questions (which were also your first ones) of how many games will O'Neal give us, and how big of a dropoff in production will we get from the big 3 are the ones that are most rationally headed south.
Condensed games, age, and the fact that O'Neal is already battling injury (the wrist injury that hampered him last season, that still pains him and requires season-ending surgery to fix) don't bode well for either problem, and that's what I'm getting at here.
If we had a legitimate starting 80 game center, I think we could manage the rest of them.
Do we even know the lineup yet? They could go RR/RA/PP/Bass/KG with a bench of JO/Wilcox/Green/Daniels/Dooling, plus they'll still sign a player or two.
I'm sure that'll happen, but that doesn't contest for the top spot in the East, and it likely doesn't make the Celtics within the top 3 or even 4 teams record-wise coming into the playoffs.
And that's assuming no major injuries.
Sure there's a shot, but ask yourself if a frontcourt rotation of Bass/KG/JO/Wilcox will give the team what it needs defensively?
Compare that to other frontcourt rotations in the East. Noah/Boozer/Gibson/Asik, Amare/Chandler, BBD/Anderson/Howard, Smith/Horford/Pachulla...
Then factor in that the only likely top 5 seed that will have a worse FC rotation features 2 of the top 5 players in the NBA.
And that's not even factoring in Rondo, Allen, and Pierce's health, not to mention KG's.
There's a shot, but without a steady presence in the paint next to KG, it is a heck of a lot longer.