It's not just that. Of course GP2 being out 3/4 of the games and not being his own self had an impact, same with Justise missing 1/2 of the games.
Hart however missed a very small number of available games.
But, It is also roster construction and misuse of the roster.
Even if these 2 and Nurkic were available, the Blazers would still be undersized - and their defensive rebound rate % is in the lower third of the league, your defense will go down if you give opposing teams additional options to extend possessions after misses, and the amount of time Dame and Ant play together means that the Blazers are vulnerable to switches from multiple positions. Add the fact that the bench is very young, young players, even if they have the athletic ability and will are prone to mistakes, we have seen it over and over with Shae, Jabari and Trendon playing even tho all 3 I am sure will be fine defenders as they mature, both Shae and Trendon have already shown willingness to work on this aspect of their game and Shae, especially, has the tools to be an elite defender.
Finally, Nurkic missed 50% of the games, but unfortunately, in the other 50% of the games we played him in switching defense or 2-3 zone instead of in drop-coverage that plays to his advantage.