There is the whole rest vs. rust debate to be had here. Game 1 is Monday, plenty of time for the Celtics to rest up, get some practice in, work on schemes for PHI. Meanwhile, the Sixers have not played since Apr 22. That will be over a week they have been off.
The rest is probably good for the likes of Embiid and Harden but that is a long time to have not played an actual NBA game. Plus, I don't think even that long of a rest is really going to cure what is ailing Embiid and Harden. Their wear and tear runs deeper than something that can be fixed with a week of rest. The rest is better for Embiid and Harden than not having rest but I am not sure that for the the team overall, that this extended layoff is going to be a net plus.
The bottom line is I think the Celtics should be fine against the Sixers. ATL is a better team than I think a lot of people gave them credit for, so I don't feel that losing a couple to them is any kind of an indication of Celtics weakness. The Celtics will not be able to do any sleepwalking though, or freeze up at the end of games.