Marc Gasol, like Bird, McHale, and Parish, is not walking through that door. DeAndre Jordan is not walking through that door. Even if Boston had the cap space (which it wouldn't after a Rondo re-sign in this scenario) there's no reason why Jordan or Gasol would pick Boston over the other teams because of the limits on how much you can offer guys. When money is equal, location and quality of team become the prime factors and Boston is lacking in both areas.
Yes, it's possible for a guy like Gasol who's a little older that Boston could overpay with a huge 5-year deal, but I don't think that's really the smartest move for the franchise at this time.
If they're really going to build around Rondo they need to get the record to a point where it's respectable and FAs/trade targets think, OK I can see this team being a perennial playoff team with me on it.
Secondly they need to acquire guys who are young and undervalued who have the potential to exceed expectations. Everybody knows guys like Anthony Davis are awesome, but not every GM would have predicted Bledsoe and Dragic having great years in Phoenix, or Millsap developing a three-pt. shot in Atlanta. Based on Rondo's age they can't be drafting all these 19-20 year olds who are years away from their prime. They need to find the guys who have been in the league 3-5 years and take a shot that they could be better than most think.
Someone like Turner could qualify because he's talented and still young but due to a disappointing season didn't find a lot of suitors in FA. Or, let's say that they take a chance on trading for Larry Sanders and the guy suddenly matures a la Zach Randolph? When the obvious stars are not available to you, these are the options you have.
The hope is that after enough good moves the team is competitive enough so that they are not laughed out of the building when making FA/trade pitches to that star(s) that will take you over the top (like how significant the Harden trade was to Houston).