They didn't take a giant step back. IT, if healthy can still get you 20-25 PPG, and Crowder gives them another 3 and D guy. They still have a ton of shooting, and I wouldn't be surprised if Wade ends up there eventually.
And they still have LeBron. He's still the best player on the court by far, and the Celtics don't have anyone to guard him. Crowder didn't even come close in May.