Sure seems like he is slipping in the rotation. I know it's a small sample size, but the team has been more effective with one big on the court and a bunch of wings cutting and shooting. With Layman and Hood playing well, and Harkless and Aminu with good games at times, this appears to be the first opportunity to push Collins back in the rotation.
Biebs is playing within himself and doing some good things. Collins just doesn't seem to be providing a consistent defensive presence and isn't hitting his shot well either. The team seems to be playing better with Nurkic/Biebs at the 5 then any combination of Aminu, Harkless, Layman, Hood, at the forwards.
I still think Collins could be a very solid player, but maybe the reality that he needs another year to bulk up and work on his moves and shot.