It's not just that Doncic came into the season out of shape, it's that he appears be putting on weight as the season progresses. The back issues are very likely related to carrying too much weight, and could lead to a shortened career if he doesn't get it under control.
Jokic was a pig last year, but came back noticeably leaner; not muscular yet, but leaner.
If only Doncic could get advice from some MVP-level player like...oh..say..Steve Nash, who elevated his level of play and extended his career (after leaving the Mavericks, unfortunately) by dedicating himself to conditioning.
Or current teammate Redick, who had to transform his non-elite physique into an NBA-decent body, and has a 15-year career to show for it.
And Dirk wasn't too much further along in his career when he started the serious conditioning, enough to go for 20 years.
Pretty much any veteran player with any longevity will say that the key to a long career is learning to take care of your body--through nutrition, through conditioning/training, through getting enough rest. You can't just play off 'youth' indefinitely.
