The Boston Celtics are 13-3 over their last 16 games and have distinguished themselves in the Eastern Conference. Spearheaded by the play of Isaiah Thomas, the Celtics are putting up absurd offensive numbers and currently own the seventh-best offensive rating in the league.

There's a lot of reasons for Boston to feel good about itself and that would include the fact that the Brooklyn Nets, whose draft spot the Celtics can swap into in June, have lost 11 straight games. Brooklyn owns the worst record in basketball by 3.5 games and ESPN's Basketball Power Index currently projects the Nets with roughly a 25 percent chance to emerge with the No. 1 pick.

But if there's one lingering concern in Boston, one thing that really keeps Brad Stevens up at night, it's the team's underperforming defense.

Boston tied for the fourth-best defensive rating in the league last season and, with the offseason addition of Al Horford, many wondered if they'd scale higher this season. But as the Celtics head to the back nine of the regular season, they find themselves trying to prevent sliding into the bottom third of the league in defensive rating.

The Celtics rank 20th in the league while allowing 105.8 points per 100 possessions. This is a somewhat unfathomable number for a team that was nearly five full points better a year ago. The team's offensive explosion can absolutely be blamed a bit here. Boston's offense was so inconsistent a year ago that the defense had to force stops in order to keep the Celtics in games.

This season, Boston has been content to trade baskets then watch Thomas go into fourth-quarter takeover mode and help his team sneak out with a win. That all but a small handful of Boston's 25 victories have been nail-biters only confirms that Boston hasn't operated with the sort of defensive intensity that could have turned more of those wins into laughers.

That's why Horford couldn't help but mention the defense when reflecting on Boston's success over the past month after Monday's win over the Charlotte Hornets.

"[The Celtics are] still not necessarily pleased with our defense, but we’re getting better," Horford said. "Defensively, we have to make sure that we keep making strides."

Boston most certainly dug itself a hole at the start of the season. Its defensive rating through seven games was a ghastly 112.3, which was more than three points worse than the nearest competitor. The Celtics seemed to settle in a bit from there and, over the next month, displayed a more familiar defensive tenacity, even if it didn't always translate to victories while going 10-8 with a 101.7 rating in that span.

Since Thomas returned from a groin injury on Dec. 16, the Celtics are 13-3. An offensive rating of 112.3 ranks them in the top 5 in the NBA over the past month. But Boston's defensive rating in that same span is 107.6, which ranks 17th.

Stevens hasn't been bashful about pointing out that his team's defense must improve. He's made sure to note that Boston's first-shot defense has typically been very good but the team has often gotten burned by second-chance opportunities created by the Celtics' lack of size and rebounding. And Stevens knows that the Celtics absolutely must be playing better defense when the playoffs arrive.

"I think by April you want to be playing your best in both [sides of the ball]," Stevens said. "Last year, I thought we played our best defense early. We made a conscious decision to go smaller later and we became a better team, as far as wins and losses and offensively, but we dipped defensively in a big way. This [defensive erosion] started at the [end] of last year. But our numbers were so good that they held up. We've gotta become a better defensive team whether we’re playing big or small."

The good news for the Celtics? Defensive rating isn't the only measure of a team's overall success. The Cleveland Cavaliers are less than a half-point better than Boston in defensive rating this season and rank 14th overall. The defending champ Cavaliers sit seventh in net rating (plus-3.6) and that's just one spot ahead of Boston (plus-2.9). Not everyone can be the Golden State Warriors, who rank second in offensive rating and first on the defensive side.

The Celtics talk a lot about playing on a string and sticking to their principles. The middle part of the season is a reminder that this team can be a very good defense when they commit. Regardless of their offensive success, Boston has to find a way to get back to playing more consistent defense, if only so the team doesn't have to fight to close out games as much as it has in has lately.

"We’ve been good at times in games but not as good as we need to be ultimately," Stevens said. "We're guarding at a high level about half [of games]. And we’re going to have to be a lot better than that if we want to be a good team."