There was a collective deep breath after the Celtics eliminated the Atlanta Hawks in six games Thursday night with a 3-point win. Those weren’t sighs of relief, however, over their next opponent.

While the Philadelphia 76ers are the eighth seed and barely made the playoffs after a collapse in the final month, they present a definite challenge to the Celtics despite their youth and inexperience. The 76ers beat the Celtics by a combined 45 points in two games at Wells Fargo Arena during the regular season, outclassing the Celtics with athleticism and defense.

The Celtics’ excuse - and it’s legitimate - is that they played the 76ers in the first meeting March 7 one night after an overtime home victory over the Houston Rockets. Philadelphia was rested and determined to hold on to first place in the Atlantic Division.

In the second meeting 16 days later, Mickael Pietrus sustained a concussion on a scary fall during the second quarter, Avery Bradley tweaked his ankle, and Ray Allen did not play because of his bothersome ankle, and the 76ers won, 99-86.

The Celtics realize that injuries and fatigue cannot be used as primary excuses for those losses.

The Celtics have historically struggled with more athletic teams and what makes the 76ers so difficult to defend when they are playing well is that Andre Iguodala, Elton Brand, Lou Williams, or even Spencer Hawes or Evan Turner can score the critical bucket.

It’s not that the 76ers are the most talented team the Celtics will face, but they have the athleticism that allows them to compete with the league’s elite.

“They want to get into transition and when they get into transition, they score,’’ coach Doc Rivers warned. “When they get into the halfcourt, then it’s a little more difficult. And that’s going to be our test. Listen, we’re not going to outrun them, they are faster than us. They are going to win the track meet.’’