I would pick the Celtics in 6 in a hard-fought series. The Heat are very good.
Spoelstra is an excellent coach. He will certainly work to adjust and give the Heat every advantage he can.
They have toughness (Butler, Tucker, Bam, Lowry, etc.)
They have shooting (Herro, Strus, Lowry, etc.)
They have elite defensive stoppers (Butler, Bam).
They have a closer/star (Butler).
In our favor, we really have no weaknesses if we play our best.
We don't really have a single defensive player to mismatch-hunt (maybe Pritchard, but he plays limited minutes and he competes). We should be able to hunt Herro and Strus at times, and they both play big roles for the Heat.
While the Heat are tough and physical, they do not have the team size of the Bucks. Tatum and Brown should both be able to get to the rim more than the Bucks series.
While Butler is an excellent player and definitely a star and closer, he is not the shooter that Tatum is. Butler is a challenge no doubt, but being able to close him out under control as opposed to having to fully run him off the 3-point line is the kind of detail that can swing a series.
Bam has always killed us. Horford is going to have to be huge against him. We could really use Timelord back as well.