One thing the Celtics have going for them is that they've had the same starting 5 for a few years now.
In his latest podcast, Simmons brought up the fact that the Mavericks changed 40% of their starting lineup midway through the season, and that was a huge factor he overlooked in predicting that series. Of course, when he went on to off-handedly assume that the Cavs would easily beat the Celtics, he didn't mention the fact that the Cavs starting lineup has changed drastically since last year. Three out of the five are different; one was added mid-season and the other was in and out of the lineup all year.
The point is, the Cav's starting five hasn't really been tested too much in difficult playoff situations as a unit yet. That could play in the Celtics' favor.