There's no denying that the Celtics have improved this season. They may have the 5th seed, but they were actually tied for 3rd in that 4 way tie with the Heat, Hawks, and Hornets. Big leap from 25 wins two seasons ago. They are looking to make another step up in their development in these playoffs. Even if they don't win the series, making it competitive and winning at least a couple of games will be a definite step up and should let free agents know that they have a good thing going here.
Of the 3 other teams in that the Celtics were battling for playoff position, they may have drawn the worst matchup for them in this series.