With Celtics’ defense slipping, it’s time for more Aron Baynes

(Maddie Meyer/Getty Images)

Al Horford is not his usual self this season, a fact that has been evident for weeks even before his knee injury became public back in November. While Horford’s scoring efficiency has been better over the last month since taking eight games off to rest his runner’s knee (18 points in Friday’s 122-116 win over the Grizzlies), the 32-year-old is clearly not where he was last year from a defensive standpoint. The Celtics were better on defense last year when Horford was on the floor, but as he has slid into a full-time starting spot at the center, the dropoff has been considerable (team allows four more points per 100 possessions when he plays this season).

Some of that dropoff has been due to his age and reduced mobility due to his knee, but there has been another underlying factor in his struggles: He hasn’t been paired with one of the best big men defenders in the league nearly as much.

Through 44 games entering Friday night, Aron Baynes (six points, season-high 12 rebounds in 22 minutes on Friday night) and Horford had played together for just 38 minutes over nine games. That’s roughly 11 percent of the total Horford and Baynes had played together at the halfway point last season (319 minutes).

There are reasonable explanations for that dropoff. Horford (nine games) and Baynes (18 games) have missed more games than anyone else in the Celtics rotation while healthy. Brad Stevens has opted for more small-ball based lineups as well in order to create more minutes for the likes of Gordon Hayward, Marcus Morris and Jaylen Brown.

Still, there’s no denying that 38 minutes in nine games is an absurdly low number for one of the best defensive duos in the league last season. Horford and Baynes allowed 95 points per 100 possessions last year, a number that was the lowest among regular duos (300-plus minutes) on the Celtics roster. This year?