I think the Celtics have a real shot in this series; I'm a little disappointed at the amount of "Cavs in 5" or "Cavs sweep" I'm seeing today, not necessarily here, but online.
I think we have to remember that the Cavaliers stumbled through the regular season, and probably should have lost to Indiana but were bailed out by Indiana not knowing how to break a press and Oladipo saving his two worst games of the season for the middle of that series. I think people are putting too much stock into the Cavs dismantling Toronto, a team that played very, very badly. Outside of Valanciunas, every player on Tornoto's team under-performed, especially their two stars in Lowry and DeRozan, and Casey was exposed as an average coach who was slow to adjust throughout the series. The hype around Cleveland right now mirrors what was being said after the trade deadline when Cleveland won a few and people were saying they were fixed; only as that sample size got bigger it was revealed that Cleveland still had a ton of flaws that could be exploited.
With the huge exception of LeBron, the Cavs rely on a cavalcade of specialists who have specific strengths and weaknesses, which contrasts with the Celtics depth of multi-tooled players. So much of Boston's success right now is based on the depth of talent they have on the floor that can all dribble, pass, shoot and defend. I think if Boston can stifle the ball movement the way they did in the Phildelphia series and prevent the Cavs from shooting a good percentage from three, they will be in great position to win the series.
I also think a key to the series will be to attack LeBron on defense. He's played massive minutes all season, and something interesting that Brian Windhorst has mentioned a couple times is that this season LeBron has really focused on finding time to rest during games. Someone mentioned earlier this week that during the playoffs, LeBron moved at the 10th slowest average speed of any player. That doesn't mean LeBron is slow, but it means that he is walking a lot and trying to conserve energy throughout the game at all times. If you watched him this season, you will definitely notice him slacking off on defense and taking offensive possessions off. The Celtics cannot let LeBron rest on defense; they have to exploit his attempts to go on cruise control and make him pay for slacking off in order to conserve his energy.
I think they did a really good job in the Philadelphia series going after Embiid when he was tired, putting him in the pick and roll and picking on his fatigue and willingness to concede open looks. LeBron is obviously a much better conditioned player than Embiid and infinitely smarter; but the Celtics best strategy to defending him is to tire him out.