I watched the replay of the moment when he was "writhing" in pain, eyes closed and covering his face with his hands. Then for a second his eyes opened, and gone was the facial expression that would have showed his pain.
He just plainly faked it. And for him to do this when the team was down and he wasn't playing well just showed a lack of leadership mentality on his part. Grow up, young man!
I also replayed this a few times and this looks like what happened:
Tatum had the ball stolen from him by Oladipo. He tried to grab back at the ball with his right arm but was unsuccessful. He fell backwards and used his left arm to break his fall. He was seated on the ground and reached over with his left hand to his right shoulder. The ball was passed to Strus who went for the dunk but was rejected by Brown. The ball fell to Horford who passed it back the other way to GWill, who pulled up for a 3 9 seconds later while Tatum was still on the ground holding his shoulder. As Tatum turned his head to watch, the shot rimmed out and Brown saved it out of bounds to Smart, who also saved it out of bounds and passed it back to Brown, around the 12 second mark. At this point Tatum lay back and started to writhe around on the ground holding his right shoulder, while Brown made a layup and the foul. Then he's writhing around for a while as the trainers attend to him. He opens his eyes for a brief moment at 0:40 then closes them again.
Here's the video replay, I'm just describing what happened on it:
https://www.youtube.com/watch?v=Wis6DE5c4m0Now one of two things could have happened:
1. Tatum lost the ball, he was sitting on the floor frustrated because he thought the Heat were going to score on the turnover, but Brown came up with the block on Strus and the ball was quickly moved the other way and GWill took the 3 which missed. Maybe Tatum didn't want to jump up off the ground when he was holding his shoulder so he started writhing on the ground while the play continued and eventually Brown made the shot. Kind of like how soccer players pretend they are injured for at least the first minute after they go to ground.
Or...
2. He genuinely suffered a stinger and genuinely couldn't get up and needed time to recover.
I suspect what happened is 1, he didn't want to make it obvious he was embellishing an injury. I think it did hurt but he probably could have got up. Just didn't want to because he thought Strus was going to make the shot so why run. Because if he was really hurt he wouldn't have looked back at the basket when GWill shot, and I would imagine he would have started writhing in pain as soon as he fell, rather than sit there for 10 sec before doing so. Then because he didn't get up when GWill made the shot, it would have looked weird to suddenly jump up.
But I will give him the benefit of the doubt that it was 2. His teammates will be the best judge of whether it was genuine or not.