A lot depends of how OKC finishes the season, and how the Celtics finish the season. If OKC get knocked out earlier than expected, and the C's extend deeper than expected, it improves our odds.
The key is to convince Durant that by switching to the East it gives him a much better chance to get to the finals. He could remain in the West and be a brides maid for 3-4 more years. The Warriors are a relativity young team.
Other than the Cavs there are no real "special" teams to contend with. Add Kevin to the Celtic roster, and add a suitable center, and we can more than contend with the Cavs. (Lebron has a ton of miles on him, he's due to begin breaking down, Love?...please, Irving can't do it alone.)
Sell Kevin Durant on the Celtic lore, the mystic, the banners hanging from the rafters, the fans. If some stars line up...we have a some chance.