With what I witnesses watching the Heat vs Celtics, There is no way in my mind that Boston even makes it a game vs the Warriors. Their only Chance is if Tatum and Marcus Smart play great offense and great defense every game. If those 2 leaders for the Celtics have a bad game, they will lose the game. They don't have a deep enough bench IMO.
Even if Tatum and Smart have poor performances, the Celtics can still hope for Jaylen Brown. This guy was pretty good in the series against the Heat. In addition, Horford can also be useful for the Celtics, especially in the paint. However, you're right that the Celtics have a bad bench. I would even say this is their main disadvantage because their coach allowed only 3-4 bench players to play in the previous series, while some players in the starting lineup played for 40+ minutes almost every game.