Before the 2016-17 season tipped off, Boston Celtics big man Amir Johnson was asked how the team might take its defense to the next level.

"Rebounding is definitely a key that we need to focus on," Johnson said. "It just needs to be a team effort on the glass. Once we get that down pat, we will definitely be an elite defensive team."

Two games into the regular season, the Celtics have been exposed on the glass and their inability to snatch rebounds is an obvious reason why their defense has looked subpar.

Boston finished tied for fourth in defensive rating last season and that was before they signed Al Horford. Throughout training camp, players were peppered with questions about whether the Celtics could emerge as the league's best defense.

What's obvious through the first two games is that they need to be better on the glass and, while Horford was an obvious upgrade in the Celtics' frontcourt, the team does miss the rebounding efforts of the likes of Jared Sullinger.

Al Horford and the Celtics have had their troubles on the boards, which was the case Thursday against Jimmy Butler, Robin Lopez and the Bulls. Dennis Wierzbicki/USA TODAY Sports

While certainly a small sample, it's a concern for Boston that the team has hauled in a mere 67.3 percent of available defensive rebounds over its first two games. For context, the Houston Rockets finished in the basement of the NBA in defensive rebound percentage last season at 72.8 percent.

The Celtics knew rebounding might be an issue. Boston is the shortest team in the NBA with an average height of 78.2 inches. The Celtics are more than a full inch shorter than the league average of 79.3 inches. Celtics coach Brad Stevens often leans on small-ball lineups, so it's not unusual for Boston to give up size on the court.

But a size deficiency doesn't explain how the Celtics have given up a whopping 33 offensive rebounds leading to 41 second-chance points in the first two games. That includes giving up 18 offensive rebounds and 18 second-chance points to the Chicago Bulls during Thursday's 105-99 loss at the United Center.

There were other factors that contributed to the loss -- especially Dwyane Wade's out-of-nowhere 3-point shooting -- but the Celtics put themselves in an early hole thanks in large part to their rebounding woes.

Stevens said after Thursday's game that the key for Boston comes down to putting bodies on defenders and letting perimeter guys help clean up the glass. Stevens pointed to instances when Avery Bradley swooped in for a rebound while Boston's big men boxed out.

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Blame tired legs on the second night of a back-to-back, but Boston got manhandled on the glass as the Bulls outrebounded the Celtics 55-36. There was one sequence in the first quarter in which both Horford and Johnson were under the basket on a shot attempt and both were haphazard with trying to box out Robin Lopez, who simply muscled his way between the two and tipped home a miss.

Boston isn't going to get much bigger. The Celtics leaned heavily on Tyler Zeller on Thursday, but he's a fringe rotation player when the team is at full strength. Boston has been playing without 7-footer Kelly Olynyk, who is rehabbing from offseason shoulder surgery, but he's a below-average rebounder even when he is on the floor.

It's condemning that, through two games, the player with the best defensive rebound percentage on Boston's roster is Gerald Green at 19.4 percent. Horford (16.7 percent) and Zeller (16.2) are the next closest but both need to be better because of Boston's lack of size. Johnson, who got moved to a bench role in the second half on Thursday's game in part because of his rebounding struggles, also needs to be better, having pulled down only 11.8 percent of available defensive rebounds through two games.

For the Celtics, it's as simple as this: It doesn't matter how good your defense is if you routinely give teams easy second-chance opportunities. Boston must make rebounding a priority for all five players on the floor.