A few thoughts.....
1) I agree Bam is a matchup problem at least on paper. I'm not sure whether TL will or won't be able to give some good minutes there---he has changed a great deal in recent weeks and I think no one really has any idea. Theis is not a great matchup, and Kanter isn't either. I don't think Grant is a great matchup, but it is not impossible he can give 15 good minutes. Bam also needs to show whether he can be a 20-25 ppg guy as a focal player for 7 games which would make it tough for Cs.
2) Toronto's core defensive challenge was that they couldn't stay in front of 4 Celtics (Smart, Kemba, Tatum, Brown) off the dribble. Miami can D up a couple of those guys at a time with Butler, Iggy, possibly Bam on Tatum. Maybe Crowder as well, not sure he's quick enough but he's very solid. But their guards are materially worse at the point of attack, with Dragic and Herro simply being bad, so they either are going to rely on a zone (which Celtics improved against vs Tor, but is still a question) or do a lot of trapping/rotation. I just don't see them being even as good as Tor was at stopping the ball, and Toronto wasn't good at it save game 6. Especially with Hayward coming back in some form at some point, I think there's a real chance this is again the biggest issue in the series---when Celtics spread and create space they end up with a lot of one-on-ones ending in layups or penetration
3) ESPN pointed out that Miami draws a lot of fouls and that FT are a bit of the offense's 'secret sauce'. The degree to which this continues---Celtics defense, sometimes-reduced whistles in playoffs, and Miami's skill at it all intersect here---is a big deal as I don't see Miami's offense as being great otherwise.
4) Butler is a guy who will always compete but not always succeed. Celtics have some good defenders for him and while I know Mia has had success in individual games without Butler I do think over the course of the series if he isn't putting up a few games in the mid-20s/low 30s it is likely Miami won't keep up offensively
5) The Celtics role players were collectively effective vs Toronto, I thought. Wanna had several good games and was bad only in game 7; Grant was solid in his role. Semi was solid overall given his limited role. TL was overall effective, even briefly impactful in two games. Especially if Hayward contributes they don't need anyone to do more than they did vs Toronto, but they do need guys not to be terrrible. So, this one is really about being passable---if they can get minutes from Wanna, Semi, Grant/Kanter that don't hurt them it's a big win. Miami has better depth, but that is largely irrelevant so long as the Celtics role guys are not negatives