I listened to Saban say this. He basically was asked why Gilmore got into the game as a receiver against Carolina. If it was a reward for his special teams play or what? Saban said, no, it is because he does good. It was kind of funny, in a Saban way of course.
Basically, he said Gilmore is the ideal replacement for Chambers for a few plays to help keep everyone fresh and allow their talented players ( like Gilmore ) to get on the field and make plays. He isn't really "taking" Boston's spot. It is just that Saban feels he needs to be on the field a little more often, and quite frankly, I agree.
People got way too hyped over Boston in preseason. Remember, most of his time came against camp fodder and depth chart players. Plus, he was used perfectly in TC and preseason. His legs are shot, we cannot expect to see him for more than a few plays at a time without risking him. The coaching staff knows this, and uses him as such. When he is in, Gus looks for him, which is all you can do for the guy. It is up to him to remain healthy and get open while on the field.
Right now, Gilmore backs up Chambers and Boston is probably more of a backup to Booker or simply just a backup like Welker. We should be fortunate to have such depth.