I think it is all about preparing Goff and while they like Mannion their focus in on preparing Goff to be the starter and he will grow and learn more as the number 2.
Keenum is there to see if they can win a few games before Goff takes over and the main goal is for Goff to be successful when he finally gets the start.
It would not surprise me if Keenum has issues in the next game or 2 or 3 that Goff relieves him and gets some time.
If Goff does well he will he will start or shows good promise he will finally take over.
Longer term Mannion will be the 2 and I do not expect them to pay Keenum 3 Mil + next year.
I think this is how Fisher saw things unfolding all the time and that he wants to keep the pressure off Goff as much as possible while helping him get acclimated to being a starting QB in the NFL.