I think the loss of Ariza has a more profound affect on our defense, than people are willing to admit.
So, people resort to blaming Monty because he has to play Rivers, Thomas, and Vasquez extremely high minutes, and all of them are terrible on perimeter defense. Thomas is at least decent in some respects, but lacks the quickness to close out shooters.
Thomas is good against 3s like Melo and Granger. The guys who are a little slower than most 3s but can post up and shoot. Thomas has a good body and defensive ability against those type of guys.