By Andrew Mooney, Boston.com Correspondent

As ESPN’s John Hollinger artfully illustrated in April, the Celtics’ dramatic turnaround from has-been, elder statesmen of the East to dangerous veterans bent on one last title run can be attributed almost entirely to their defensive resurgence. The introduction of Avery Bradley into the starting lineup and the shift of Kevin Garnett from power forward to center transformed a .500 team into a dark horse Finals pick.

The numbers back up the wisdom of this lineup shift. With what was the team’s most common five-man unit of Rajon Rondo, Paul Pierce, Brandon Bass, Kevin Garnett, and Ray Allen, the Celtics’ defensive efficiency (number of points allowed per 100 possessions) was 98.9 during the regular season—solidly below the league average of around 102, but still unspectacular, given that the unit was only just above the league average for offensive efficiency (103.4 points per 100 possessions).

But, after Allen’s nagging ankle injury caused Bradley to be inserted into the lineup, the Celtics have been a far more productive team, on both ends of the court. The unit of Rondo, Pierce, Bass, Garnett, and Bradley allowed just 89.8 points per 100 possessions, almost a full six points better than the team’s overall average (95.5). They have also been a superior offensive unit, averaging 109.4 points per 100 possessions, powered by a highly efficient shooting percentage (50.6 percent) from the field.

However, though this lineup is capable of locking down an opposing offense, it remains plagued by the one aspect of defense in which the Celtics have been notably deficient all year: rebounding. This is not a problem unique to this five-man unit, but rather due to the team’s construction; the Celtics' relative lack of size in the frontcourt makes them susceptible to getting pounded on the offensive glass, an issue that has increasingly come to light this postseason.

Friday’s Game 4 against the 76ers was particularly representative of this phenomenon. Though the Celtics held Philadelphia to 37.8 percent shooting from the field, the 76ers attempted 11 more shots from the field, in large part because of their 17-5 edge in offensive rebounds. As the Celtics discovered, stingy defense becomes less of a weapon when allowing more possessions each trip down the court.

In fact, the Celtics’ fortunes during the playoffs have closely followed their performance in the rebounding battle. In their series with the Hawks and now the 76ers, the Celtics have won every time they have outrebounded their opponent (5-0). When they have trailed their opponent in rebounding margin, however, the Celtics are just 1-4, with the only victory coming in the Game 4 blowout of Atlanta. In those four losses, the Celtics have been outrebounded 190-148, and in only two of these 10 contests have the Celtics held the advantage in offensive rebounding margin.

So far, the Celtics have been able to get by despite their futility on the boards due to their otherwise outstanding team defense; in ten postseason games, Boston opponents are shooting, on average, just 40.51 percent from the field. But, as happened Friday, much of that advantage is nullified when giving up a significant edge in offensive rebounding, and thus, additional attempts at the basket. Tied at two, the series looks to be a toss-up heading into Monday’s pivotal Game 5. The result may hinge not on the offensive production of Kevin Garnett and Brandon Bass, but on the ability of the frontcourt to clean the glass and prevent second chance points.