I think the only East team that can push a healthy Celtic team is Philly if they get Kawhi. However, it's been said SAS want Simmons for him, and a Kawhi/Embiid combo with the rest of their players isn't scary.
Regardless of how they got him, I think the only way that team can beat us is if they get Kawhi, keep Simmons, and Simmons improves his outside game dramatically.
Even so, we have a deeper, more all around talented team than the East has had since Miami at their best with LBJ, imo. The CLE teams were Lebron and Kyrie with a bunch of role players. We have the potential to throw 4 or 5 very good scorers at anybody, with good perimeter D. Given how Brad Steven's teams have given GSW fits the last 4 years, I really like where we are if we're relatively healthy headed into the playoffs.