anything can happen in the draft, but i'm pretty sure Rondo-Bradley-Green-Sully will not win a title. And to the person saying no one expected the Pacers, well George and Hibbert are both very legit (and underrated by most non-Pacer fans).
the only time that celtics "core" will contend is if the aforementioned Center is indeed an all-star caliber center on the level of Rondo. So yes, a core of Rondo-AllstarCenter can contend, especially if backed up by solid role players like Bradley and Green and Sully. Green is a very solid 3rd option imo and Bradley is the defensive specialist every aspiring contending team needs (like TA or D. Green or Sefolosha or Posey or Shumpert or Battier or Ariza etc.)
But again, we'd probably need a prime Shaq, Hakeem Olajuwon or Tim Duncan first.