Look, the Cavs are (and should be) the clear favorites. They clearly have the best player in the series, and their second and third best players are right on par with our first and second best players. They're also rested, though that will show quite a bit of rust in the first game.
However, all of this talk about us for sure getting swept is incredibly premature.
There are several reasons to believe that this will be a competitive series:
1) Unlike Washington, we have several places to hide IT defensively, especially in the starting lineup. That was the biggest factor holding back our defense against Washington, because Wall, Beal, and Porter (though Porter did much less than the others, making it questionable why we didn't go that route for longer) all took advantage of IT defensively and really abused him. JR Smith and Shump are not the types of players that will be able to use their size over IT effectively, and it's a major win for us if they try.
2) Cleveland's defense, especially perimeter defense, is nowhere near the level of Washington's perimeter defense. Washington had success in trapping and double-teaming IT off of PnRs, which made him trust his teammates. But Kyrie, Smith, Love, and TT are not nearly as efficient in that regard as Wall, Beal, Morris, and Gortat. TT will be the only adequate big that can attempt to try and help contain IT off of PnRs, but he still won't be able to stop IT from imposing his will.
Furthermore, we should be able to take advantage of poor rotations and off-ball defense even more against the Cavs than we did against the Wiz, which is important because we got a ton of points off the Wiz that way. Cleveland also only has one defender who has any sort of chance of staying in front of IT and containing him in Shumpert. Washington, on the other hand, could throw Wall, Beal, and Oubre at IT and give him different looks defensively to make him work.
3) The IT/Al PnR with Kyrie/Smith and Love will be a bread and butter play that Cleveland is going to have a really, really difficult time of stopping.
4) Where the Wizards are much more oriented toward attacking the paint, getting to the rim, and using the midrange game, Cleveland is a much more perimeter-oriented team who shoots a ton of threes, which works out well given that our defense is predicated on making it difficult on opposing shooters.
5) Most importantly, though, the biggest advantage that this team will have over the Cavs is home court advantage and being the underdog. While we were considered the favorites over Chicago and Washington (even if slightly so over Washington), nobody will consider us the favorites over Cleveland. This works well in our favor, because this team has consistently showed throughout the year that we play our best ball when our backs are up against the wall, we're being counted out, and we engage that chip on our shoulder. Unlike against Washington, I expect us to give Cleveland our very best in effort and intensity every night, and I expect all of the games to be dogfights.
Now all of this isn't saying that I think the Celtics will necessarily win the series. I think it's certainly possible for us to win, but a lot would have to go right for that to happen. However, I do certainly think that this will be a very competitive six or seven game series and that we make Cleveland actually work for the wins.
I also foresee a big series for IT. I just don't think that they have the pieces to contain him and will focus on stopping the rest of the supporting cast.