It's not just two excellent defensive teams squaring off that point to an Under in this matchup. Both rank among the top seven teams in defensive efficiency. 
This is a division rivalry. It's a very early start time - noon East Coast time. And the game is being played Christmas Day. All those factors are pluses for the Under.
So is the situation for Toronto. The Raptors are minus three key injured players - Pascal Siakam, who is their leading scorer at 25.1 points, Marc Gasol and Norman Powell. The Raptors overcame a 30-point second-half deficit to stun the Mavericks this past Sunday. They then went into overtime against the Pacers on Monday. This almost is like a third game in three days with the early start instead of third game in four days. Look for the Raptors to slow things down because of their heavy fatigue factor.
The Celtics have been playing at a slow pace ranking 27th in tempo during their last 10 games.