Boston have picked the worst possible time to fall into a mini-slump, with the Atlanta Hawks pushing past the Celtics 123-116 at Philips Arena on Thursday.
Leading the way for the struggling Hawks was Paul Millsap, with the 32-year-old veteran going for a team high 26 points, while collecting 12 rebounds for a double-double off the bench.
With the victory Atlanta now has claim to the fifth seed in the Eastern Conference, currently by half a game over the Milwaukee Bucks, who lost on Thursday.
The Hawks have yet to clinch the post-season, with the gap between the final five teams in the play-off race still only two games from fifth to ninth.
Atlanta couldn't be stopped on the offensive end during the first half, with the Hawks scoring 71 points heading into the break.
Over on the Boston side Isaiah Thomas finished off the night scoring a game-high 35 points, while grabbing five rebounds and dishing out four assists.
With the loss on Thursday it appears more and more likely that the Celtics will not take the number one seed away from LeBron James and the Cleveland Cavaliers.
Boston can still finish anywhere from first through fourth in the Eastern Conference post-season race and are one and a half games behind Cleveland for the home court advantage.
Atlanta heads to Cleveland on Friday to take on the Cavaliers at the Quicken Loans Arena, with James and company having won 12 of the past 13 meetings between the two sides.
Meanwhile, the Celtics will travel on Saturday to Charlotte in a must-win game for the Hornets if they are to have any hope of staying alive in the Eastern Conference play-off race.
Boston are now 40/1 to win the Eastern Conference title, with the Cavaliers 1/500 and the Toronto Raptors 5000/1.
View all Basketball lines