I don't think the Celtics can do anything to realistically be considered one of the favorites, but the team still has the proverbial puncher's chance. I still expect the Celtics to win their division.
Let's say the Big Three don't fall off a cliff, Jeff Green plays well enough to be the NBA Sixth Man of the Year, a rookie makes NBA All-Rookie Second Team without gaining playing time through injury to one of the starters, and a combination of Jermaine O'Neal being healthy and/or getting surprising effectiveness from a free agent pick-up creates adequate production from the center spot.
Individually, all those have a decent chance of happening, but it's much less likely for all of that to happen. But a string of breaks like that could occur and, if they did, a title is very possible.