I definitely agree he needs time and not being on the field does not indicate any failure as a pick. If we get Nate back he has even more time to develop, but I don't think he needs to have training wheels, and I would like to see him play in games this year, not so I can judge him as a success or bust, but so he can speed up his learning.
Acknowledging his particular situation, at the same age from the same school at the same position, Nate played 16 games as a rookie (for a team coming off a 8-8 season and dropping to 3-13, so not a perfect blueprint) and had 53 tackles, a sack, and three picks. I'm not saying Ashton needs to accomplish that, but DB is not the QB position and does not require years of grooming.
Also, even if the team can't focus on him catching up at the expense of others, you certainly hope and expect that he has been able to make a great deal of progress understanding the system since he has been back with the team.
So again, in full support of Ashton and with full patience (because I want him to help us win a Super Bowl in the next five years a lot more than I care about him contributing this season), I think it is reasonable to expect that he has had some time to study and practice and close the knowledge gap with KT somewhat, allowing his physical talent advantages to cover some more of the gap. I would really like him to get on the field in the next four weeks for his own development, and so we can all say "wow, this kid has skills" and get excited.
If we re-sign Nate and have Nate and Ashton at CB, with McGee as nickel (and returns obviously) and KT as a backup, and have Dante and Ko at safeties, I think that is the kind of secondary that can be a key piece of returning to the playoffs and winning a Super Bowl, and other than expensive veteran Nate is a tremendously young and cheap secondary as well. Go Bills!