Theoretically, it's as well possible, however, they'd have to lose all of their remaining games while we'd have to win all of ours. It isn't impossible considering that Atlanta has a very tough schedule left. I see a major problem there though, Celtics have their work done already by winning the division, which means they can start preparing for the postseason and rest their major players. That's why I predict an easy W for the Hawks who have yet to seal their playoff spot.
But looking at Orlando, it seems more probable that we reach the 6th seed, their next two games are Utah and Denver that are fighting not only for a higher seed but also to not fall out of playoff contention while we have to face Cleveland, which should be "very winnable" and Atlanta that will probably have their seed sealed by then. I can see us winning the next two and Orlando losing twice in a row, but it all comes down to our game against the Clippers and theirs against Memphis. LAC are still fighting to win Pacific while Grizzlies can smell the Mavs being just 2.5 games away.