I agree Isaiah looks more durable than ever.
Maybe the difference between before and now is that he has fully entered his prime and is blossoming in historic style. He is more sure of himself when driving to the hoop. More in control of his body. He may only be five foot seven as one of the t.v. guys suggested, but he is definitely built like a tank for a little guy. Muhammad Ali had a rock for a stomach. Punches to his gut did little damage. Maybe Isaiah with those huge arms and shoulders just doesn't get hurt too much falling on them. And as a little guy, he doesn't end up with the problems tall guys get putting too much weight on long legs.
Maybe the big move has been playing him a lot more as shooting guard. That's less energy spent dribbling up the court. It's also enabled Smart to improve on ball handling and distribution which he had to because we are still waiting for Marcus to shoot better. Although his great run at free throw shooting has certainly bumped up his ceiling.
We are built as a system which breaks ceilings. It happened with Jordan Crawford and Evan Turner, and we see it happening with nearly every Celtic, even with great talents such as Thomas.
The fact is that the Celtics as structured facilitate internal growth and development. All the clamoring for trades misses this central point. Look at Jaylen Brown, Smart and Olynyk. They are getting better month to month. So in a way we are making trades all the time without making any. We are always improving, so we are constantly trading out Brown, Smart and Olynyk for improved versions, and we still have 34 more games for such internal development.