In a game like the Houston game when Mozgov only plays 18 minutes and isn't in foul trouble, there is no reason to play Perk. Even if Mozgov had played 25-30 minutes, there would have been no need for him. Tristan gives the team a lot more as the backup center against teams that aren't using a conventional center, or arguably even teams that are using only one conventional center, unless Moz gets into foul trouble.
I just don't think Perk is of much value against a lot of teams so long as everyone is healthy. That's why I thought it might be a good idea to find a hybrid PF/C who could play in more situations (like KG or Plumlee), but that type of player/trade wasn't available.
Perk was getting too many minutes for years in OKC. Finally, last year they found another center who could play in Steven Adams and gradually his minutes came down.