The Boston Celtics are not good at rebounding, and it is not a new problem.

Last season, the Celtics ranked 26th in the league in rebounds per 100 team possessions managing only 42.2, per NBA.com. The only teams worse than them were the 76ers, Kings, Bucks and Mavericks.

They ranked 24th in rebounds allowed per 100 possessions, giving up 44.8. The Celtics’ rebound differential, the difference between team rebounds and opponent rebounds per 100 possessions, was a terrible -2.6. That was the fourth-worst differential in the league. Looking at their rebounding woes in terms of total rebound percentage, the total percentage of rebounds that a team gained based off the number of available rebounds, is just as bad. In the 2016-17 regular season, the Boston Celtics managed to grab 48.5 percent of available rebounds. Only 30.3 percent of the rebounds they got were contested, the 21st in the league.

This is not a new trend. Over the last three seasons, the Boston Celtics have ranked 27th, 21st and 21st in total rebound percentage.

The league leader in individual rebound percentage last year was Andre Drummond, who was responsible for 25.2 percent of all rebounds when he was on the court. Among guys who played at least 10 minutes per game in at least 50 games, the Celtics three best rebounders by percentage were Kelly Olynyk, Tyler Zeller and Amir Johnson, ranking 71st, 74th and 78th respectively.

The best rebound percentage by a returning player is Al Horford, good enough for 96th-highest last season grabbing 11.7 percent of rebounds when on the court. The second-best percentage is point guard Terry Rozier, who ranked 131st in the league and grabbed 9.8 percent of all rebounds. The average NBA player grabs 10% of rebounds when on the court and the Celtics are returning one qualified player from their team last year who managed that.

This matters because over the last three seasons the Boston Celtics have given up an average of 12.5 second chance points per 100 possessions, ranking 29th last year with an even worse 12.8 per 100 possessions. They themselves scored the 27th most second chance points at just 10.8 per 100 possessions last season. No team had as large a differential between second chance points scored and second chance points allowed in the 2016-17.

So, what have the Boston Celtics done to address their rebounding issue? Well, not much. They signed Aron Baynes to a one-year deal with $4.3 million. Baynes should really help on that end when he is on the court. Last season Baynes grabbed 15.8 percent of all available rebounds, the 42nd-highest among qualified players. With Andre Drummond off the court, Baynes managed a slightly better 16.0 percent.

Jayson Tatum, the third overall pick in the 2017 draft, was an above-average rebounder in college, but does not project to be anything special at rebounding in year one. Per my player projection model, Tatum is expected to grab about 9.2 percent of all available rebounds. That would have ranked 143rd among qualified players last season and is equal to how sophomore Jaylen Brown performed in his rookie year.

Guerschon Yabusele, a 2016 draft and stash prospect, projects to be an above average rebounder next year after grabbing nearly 17.7 percent of all rebounds in China, per RealGM.com. Yabusele managed 13.0 percent in eight summer league games last summer.

My model projects the Celtics to grab just 48.8 percent of all rebounds next season, the third-lowest projection. They are going to lose the battle of second chance points too. Poor rebounding is a problem that can be exploited in the playoffs. Tristan Thompson regularly kills them on the glass by just rebounding hard. The average opponent last year rebounded +0.4 percentage points better against the Celtics than any other opponent.

The Boston Celtics do not need to be a good rebounding team. They will not be next year. But they need to be competent.