Cleveland will win the East. Short of a LeBron injury, I don't think anybody in the East can beat them in a 7 game series.
As for who plays them in the ECF, it could be any of the other teams in the East. Nobody has separation from the rest of the pack. So the Celtics have a 1 in 7 opportunity. 14% odds of making the ECF.
If they hold onto the 3rd seed, that 14% goes way higher because 1) that'd necessarily mean a significant separation in results from the 6th-8th seeds 2) they'd get home court for a round 3) they wouldn't have to face Cleveland until the ECF.
If Ainge can acquire a star, I agree that Boston becomes that #2 team in the East and a strong favourite to make the ECF. I'd say they will still be short of talent to beat Cleveland though.
If there's a team in the league that best fits the once-or-twice-a-decade species of team which exceeds the sum of its apparent individual talent and beats some teams more talented on paper, it'd be these Celtics. The late 70's Sonics, the early-mid 80's Bucks, the late 80's Pistons, the 1999 Knicks and the 2000 Pacers, the mid 00's Pistons...teams more or less like that.
Could also be that five or ten years from now, if one or two of our players establishes himself as a legit heavyweight star, then our current talent level might look way more impressive in retrospect, and if we make it to the ECF or Finals this year then future analysis might be like, "Well, it totally surprised people back
then, but looking back they
did have [Marcus Smart or Kelly Olynyk or Jae Crowder or whoever] just before he became a household name and perennial All Star", etc.