IMO, a little of both. Players are worn down under Thibs -- it only stands to reason. And tired players make mistakes...or over-used players get hurt and under-used players are forced to replace them, which would result in more mistakes. At the end of the day, it definitely has to do with who he CAN put on the floor...but I don't buy the argument that Thibs is blameless in this (not that you're giving that argument.)
Thibs is not perfect, and yes, I'm not giving that argument. He still is iffy to pull the trigger on under-developed players to get them game experience.
I don't fully buy into the modern(key word,
modern) argument of players getting worn down. :shrug: Marathon runners go for 2+ hours(train almost as hard, or harder sometimes), multi-athlete competitions go for hours as well, and these people train and compete. Football used to be an ironman format. Basketball rosters were smaller because starters would play more(in Chuck Taylors and with the poor sports science to understand how to boost recovery, conditioning, nutrition) than what Thibs does. None of these examples are what I am advocating for modern basketball to be, but we know these guys are still far from so many comparisons of what really is, wearing someone down.
To determine what wears down a player, is all judged at an individual level. A good argument can be made, that a lot of guys that were good to go, were forced to sit by coaches decision or trainers. Which basically means out of the natural events of the season, some of the injuries were over-reported. Something different from years past(yes, humans can learn from their mistakes and adjust). If Thibs did not improve on his player rotations, then I would be 100% with you, but the fact is, he has already made a lot of progress, to the point that I don't even think it's worth mentioning, and to take it to another level, to the point where I hope he isn't afraid to give key players big minutes in the playoffs. Catch22 in the works, damned if you play the guy, damned if you don't.
For example, Luol led the league in MPG at ~39mpg. But historically, that is 2-4MPG lower than everyone else that led the league in MPG in the last decade, and even more MPG lower than the decade before. He relied on Luol, used him too much, but not SOO much, that it became a serious and ignored problem. Also, Thibs trend with playing his captains high minutes has adjusted, so we can see that he is fully aware that he had to adjust his own logic of how he distributes player's gametime. And this season, when going on a skeleton crew, in times where it would be far more understandable to give guys almost 48MPG, Jimmy Butler in a pinch style, he has backed off goto guy usages.