There's been a lot of back and forth in various threads about how and why the Celtics struggled against the Miami zone. It goes without saying - they could have and should have done better. But I am more on the side of the "players just need to hit shots / execute" explanation than taking issue with the scheme or approach, with the exception of the early shot clock pullup 3's raining down from Jaylen Brown and a few others in game 7.
To start, I picked out some plays from game 7 that I felt were representative of what was going on. These are in chronological order so they are not very well organized but I put my commentary besides the link:
Play 1: Here we see exactly what some were clamoring for against the zone, in particular a flash to the high post from Al Horford, followed by a Jaylen Brown back cut everyone wanted. Duncan Robinson feels the cut even without seeing it and is able to get a hand in and break up the lob. Verdict: Mostly good defense (pass from Smart should be higher).
Play 2: Again, Horford is in the high post and they hit him after Miami leaves two on the ball for no reason. Horford kicks out to Smart for an open corner 3. This is good offense and exactly how you attack the zone, but you get 0 points for it Verdict: Good offense, bad shooting.
Play 3 : This possession we see Miami in man, but I wanted to point out another instance where good offense led to zero points. They do a good job attacking early which gets the Heat on their heels. Horford kicks out to a wide open corner 3 this time for Brown. Clang. Verdict: Good offense, bad shooting.
Play 4 : This is what we saw from the Celtics all season. The Heat are in the zone and we get a kind of awkward cut from Tatum, but Martin follows and it leaves space for White to catch a ballwatching HIghsmith off guard. Paint touch -> kick out -> open 3. Verdict: Good offense, good shot.
Play 5 : Heat in the zone again. The Celtics set a pick up top, apparently that is a no no to some of you? Anyhow, its an easy pass to Horford wide open in the corner for a 3. The Heat don't even contest the shot. Miss. Verdict: Good offense, bad shooting.
Play 6 : Transition offense. Butler gambles on Tatum which leaves Horford wide open for an above the break three. Clang. Process wise I love seeing quick decisions from Tatum. Good offense, bad shooting.
Play 7 : The Heat are in man here but I wanted to show this one because Horford was killing me with this shit. Look at this screenshot and tell me why he is looking to pass no matter what. He has Gabe Vincent on him. There is no big on the floor who will contest.

Play 8 : Heat in their zone here and this is really the only egregious example I find of "potted plant" syndrome, but even that is a bit of an exaggeration. Horford gets it at the nail, this pleases some people. Jaylen makes a half hearted cut, Horford goes one on one. Bad offense.
Play 9 This should be getting the haters excited - the Heat are in zone and the Celtics high-low Bam with simultaneous cuts from Brown and Timelord. Easy 2. Good offense.
Play 10 Again, process wise this is what people are asking for against the zone. We got a flare screen from Tatum on Brown, and then Tatum goes right to the nail. Wide open foul line jumper. Is this the best shot? Probably not, its not "his" shot. But its a wide open jump shot for a scorer. He misses. I would call this Good offense, bad shooting.
Play 11 The clip cuts it out but we see a baseline cut from RW to get into a weak spot of the D and then Tatum is lurking at the nail for a wide open layup. He misses. Again, the approach is great. Good offense, inexplicable miss.
Play 12 Sometimes, it is ok to be potted plants against the zone. Here they are just swinging it around the perimeter until the Heat commit two to the ball. This leads to Tatum getting an opening and then kicking to Smart for a wide open corner 3. Miss. Good offense, bad shooting.
Play 13 Simple. Our best player gets a high screen, the Heat are in zone but weirdly playing real high on Tatum. He gets an easy opening to the rim because Marcus is in the corner and Bam stunts to him for a split second. Dunk. Good offense.
Maybe yall will quibble with my diagnoses here but the Heat ran what, 25 zone possessions in game 7? This is nearly half of those and to me I see a lot of good offense, a lot of cutting and attacking the nail that everyone loves, and a lot of missing wide open shots.
I left off four or five instances where the Heat were set up in the zone and Jaylen / White / Brogdon unleashed early shot clock wing threes because why exactly? I am sure that is not coming from the coach, it felt like everyone wanted to be the hero. I personally don't see a team that struggles with the zone or has no idea how to beat it. I see a group of guys who, whether by nerves or bad luck or injury, could not bury a shot to save their lives.
To start, I picked out some plays from game 7 that I felt were representative of what was going on. These are in chronological order so they are not very well organized but I put my commentary besides the link:
Play 1: Here we see exactly what some were clamoring for against the zone, in particular a flash to the high post from Al Horford, followed by a Jaylen Brown back cut everyone wanted. Duncan Robinson feels the cut even without seeing it and is able to get a hand in and break up the lob. Verdict: Mostly good defense (pass from Smart should be higher).
Play 2: Again, Horford is in the high post and they hit him after Miami leaves two on the ball for no reason. Horford kicks out to Smart for an open corner 3. This is good offense and exactly how you attack the zone, but you get 0 points for it Verdict: Good offense, bad shooting.
Play 3 : This possession we see Miami in man, but I wanted to point out another instance where good offense led to zero points. They do a good job attacking early which gets the Heat on their heels. Horford kicks out to a wide open corner 3 this time for Brown. Clang. Verdict: Good offense, bad shooting.
Play 4 : This is what we saw from the Celtics all season. The Heat are in the zone and we get a kind of awkward cut from Tatum, but Martin follows and it leaves space for White to catch a ballwatching HIghsmith off guard. Paint touch -> kick out -> open 3. Verdict: Good offense, good shot.
Play 5 : Heat in the zone again. The Celtics set a pick up top, apparently that is a no no to some of you? Anyhow, its an easy pass to Horford wide open in the corner for a 3. The Heat don't even contest the shot. Miss. Verdict: Good offense, bad shooting.
Play 6 : Transition offense. Butler gambles on Tatum which leaves Horford wide open for an above the break three. Clang. Process wise I love seeing quick decisions from Tatum. Good offense, bad shooting.
Play 7 : The Heat are in man here but I wanted to show this one because Horford was killing me with this shit. Look at this screenshot and tell me why he is looking to pass no matter what. He has Gabe Vincent on him. There is no big on the floor who will contest.

Play 8 : Heat in their zone here and this is really the only egregious example I find of "potted plant" syndrome, but even that is a bit of an exaggeration. Horford gets it at the nail, this pleases some people. Jaylen makes a half hearted cut, Horford goes one on one. Bad offense.
Play 9 This should be getting the haters excited - the Heat are in zone and the Celtics high-low Bam with simultaneous cuts from Brown and Timelord. Easy 2. Good offense.
Play 10 Again, process wise this is what people are asking for against the zone. We got a flare screen from Tatum on Brown, and then Tatum goes right to the nail. Wide open foul line jumper. Is this the best shot? Probably not, its not "his" shot. But its a wide open jump shot for a scorer. He misses. I would call this Good offense, bad shooting.
Play 11 The clip cuts it out but we see a baseline cut from RW to get into a weak spot of the D and then Tatum is lurking at the nail for a wide open layup. He misses. Again, the approach is great. Good offense, inexplicable miss.
Play 12 Sometimes, it is ok to be potted plants against the zone. Here they are just swinging it around the perimeter until the Heat commit two to the ball. This leads to Tatum getting an opening and then kicking to Smart for a wide open corner 3. Miss. Good offense, bad shooting.
Play 13 Simple. Our best player gets a high screen, the Heat are in zone but weirdly playing real high on Tatum. He gets an easy opening to the rim because Marcus is in the corner and Bam stunts to him for a split second. Dunk. Good offense.
Maybe yall will quibble with my diagnoses here but the Heat ran what, 25 zone possessions in game 7? This is nearly half of those and to me I see a lot of good offense, a lot of cutting and attacking the nail that everyone loves, and a lot of missing wide open shots.
I left off four or five instances where the Heat were set up in the zone and Jaylen / White / Brogdon unleashed early shot clock wing threes because why exactly? I am sure that is not coming from the coach, it felt like everyone wanted to be the hero. I personally don't see a team that struggles with the zone or has no idea how to beat it. I see a group of guys who, whether by nerves or bad luck or injury, could not bury a shot to save their lives.