I've been asking this for a while, and after a lot of consideration I think I could put together Stotts' response (if he actually wanted to tell the truth, which he shouldn't frankly):
1. You play your best players at their natural positions. All the advanced metrics over Aminu's career show he's better as a stretch 4 than a small forward. Not just this year, but past years. Collins right now isn't as good as Aminu. He probably will be in the near future, but not right now.
2. The NBA is trending toward small PF's. Starting Aminu reflects this trend.
3. Portland's offense relies on spacing. Pulling out larger PF's with Aminu's 3 point threat creates lanes for Dame and CJ. Collins hasn't demonstrated consistent perimeter shooting in game time.
4. Collins fouls a lot. I mean a LOT. I'm not giving him loads of PT until he resolves this, because guys who foul a lot often develop a reputation for doing so and it's harder to kick with refs.
5. We started Vonleh for over a year, and look what it got us. We're in win-now mode with Dame in his prime and trying to get home court in the first round. If we win more and maybe get out of the first round, we become more attractive to other players, and they'll be more interested in playing here as free agents. Maybe we get Carmello this last off-season if we'd had a better record than OKC last year.
6. We already start a pretty slow center. We sag Nurk on pick and roll defense to keep him near the rim. Aminu makes up for a lot of that by allowing lots of switching ability and quick help defense. Collins isn't as used to switching at an NBA level.
Note: I don't agree with this reasoning, but it's a pretty valid stance to take.
Collins has taken 10 three point attempts over the past 5 games, so I think there is a message he's hearing that he needs to space the floor if he wants more minutes. Also, remember,
LaMarcus Aldridge had his minutes and starts scrambled all over the place in his first season too.