If the Celtics actually play up to their level this season and get a top 1-3 seed
This is way too generous. For instance, if the C's are the #3 seed, it probably means Philly and Milwaukee are 1-2. The Bucks have 3 All-Star caliber players in Giannis, Middleton, and Bledsoe (I know there's a few people out there who thought Bledsoe should have made the All-Star team over Middleton last year). And the Sixers have 3-4 (Embiid, Simmons, Harris, Horford). It's hard to reason the C's get 4 if Sixers and Bucks don't each get at least 3. Now you have 10-11 out of 12 spots going to just 3 teams, and you still have Irving, Griffin, Lowry, Butler, Beal, Vucevic, Siakam, etc. Not going to happen.
The only way I can see this happening is the C's need to be #1 by a wide margin, we're talking at least a 5 game lead at the All-Star break/60+ win pace and you also need a few injuries along the way to other players (partially already there with the injuries to Durant, Oladipo, Wall thinning the herd).
So C's need to blow expectations out the water, Sixers/Bucks need to underwhelm, and then add in a few injuries to other players in the East.
When it's happened in the last 20 years:
2018 Warriors - 3 game lead in West, 65 win pace
2017 Warriors - 2.5 game lead in West. 70 win pace
2015 Hawks - 7 game lead in East, 67 win pace
2011 Celtics - 3 game lead in East. 63 win pace
2006 Detroit - 10.5 game lead in East, 68 win pace