He struggles in general because he's 20, but if you wondered why there parts of last night in specific that might have explained his worse-than-usual 4th quarter, the increased offensive load goes some way towards that. More than one thing can be true at the same time.
We need to retire using offensive and defensive ratings for individual players. I've done it in the past, and I've been doing it less recently because it's just not applicable: it relies on the lineups you play with. This has been a known fact for a long time, and it's why individual player O/DRtg should be totally abandoned in general: as for a single game version of that you might as well consult the bones and tell me what the prophets say. It's about as useful.
Or, in more plain language:
Defensive Rating is not an individual stat
— Anchorage Man (@SethPartnow) February 7, 2020