It has been discussed ad nauseam that the Celtics' biggest weakness on defense is their lack of presence on the interior. What has been underdiscussed is that the Raptors are ill-suited to take advantage of that weakness.
Gasol and Ibaka want to be jumpshooters at this point in their respective careers. VanVleet and Lowry aren't terribly efficient inside the arc (though Lowry can often make up for it by getting to the line).
If they want to get any offense going on the interior, it basically has to be Siakam. But Siakam's offensive game is fairly limited. His favorite way to get shots inside is to just back his man down far enough to pivot right into a layup. But that has some problems. First, it's predictable. Smart tagged him with an offensive foul by just taking the bump and going to the floor. And second, if he can't back his man down then he doesn't really have a plan B. Ojeleye largely shut him down because Siakam couldn't move him in the post. He lacks the handle to turn around and face up, and he had no confidence in his outside shot tonight.
The Raptors will have to figure out a way to generate some easy buckets in the paint going forward.