He's a terrible defender
The Phoenix Suns improved their center position by trading Deandre Ayton for Nurkic. However, the team's interior defense did not improve with the change. Nurkic, a European center, is not known for his defensive skills. Even during his tenure with the Portland Trail Blazers, it was widely known that he was not a defensive threat.
Nurkic is allowing opponents to score 28.3 points in the paint, which is not the worst for the team. However, as a center, the team expects more from him on the defensive end.
Additionally, he is slow-footed and often gets beaten by perimeter players off the dribble. It's worth noting that slow-footedness is a common weakness for big men, making it challenging for them to defend outside the paint.
Unfortunately, Nurkic is not a strong defender in almost all areas. Therefore, opposing teams may target him during the playoffs.