I don't want the Celtics to spend that type of money on the complimentary pieces until the star core is in place.
This.
Generally I'd agree with this. However, cap space is overrated unless you have some specific FA target in mind. What you need are guys on tradeable deals. Not being an expert on either Hayward or Stephenson, it seems like they could still be third bananas which would mean 10-11 million wouldn't be an overpay.
As long as you have deals that are tradeable and not all long-term ones, you can maintain flexibility. I would be more worried about signing someone in the 10-12 range this summer if say, Green and Rondo were locked in for 3-4 years and it would be tough to deal them if some star becomes available.
Not that these things can't change or aren't contingent upon a thousand different variables, but signing someone like a Stephenson or Hayward at that salary range is kind of like getting a younger Rondo or Green on a different peak timetable. That is, a player you'd feel comfortable being a third banana making 4-5 million less than your two max-level superstars.
If the draft is kind to us this season, maybe the next superstar we get will require a few years. If the draft or pick we have is overrated, maybe Ainge shifts course like in 07 and deals it for more of a win now type. Either way, a 23 year old Hayward or Stephenson would be able to fit in age-wise with both a rebuilding team OR a win now team.