The physical is part of the reason they are losing these games, but not the only reason. If you look at the Celtis through the first 29 games of the season, they consistently beat there opponenents in assists, rebounds, fg% and points in the paint. In these last 6 losses, the opponent has beaten them in at least one, and you usually more than one of those categories.
The assists have to do with ball movement and easy baskets. Before, the Celtics would play great defense and force players to go one-on-one and take bad shots. Now, holes are opening up and giving claer lanes to the basket with the ability to dump it off for an easy two.
This is also a reason why the opposition is shooting a better % from the field. They're not settling for bad shots, because they know if they move the ball around a little more, and attack the basket, they'll get better shots.
The rebounding definitely has to do with physical play. The opposition is getting to the balls first, and out-muscling our players.
But the most glaring thing to me has been points in the paint. This has been the Celtics staple. They would always outscore the opponent in this area, and usually by a huge margin. This was because they would look for the best shot on offense, but more importantly, they took pride in not allowing any easy baskets. This is no longer the case, as evidenced by all the dunks and layups they've been giving up lately.
As has been said time and time again, it all starts on the defensive end, and it's no coincidence that these losses are coming at a time when the Celtics are not playing there type of defense.