BB's press conference about the Bills at the beginning of the week discussed this situation, and not just in reference to this week. Mostly it concerned the decision to have three quarterbacks, and discussed the problem of having just one when your first-string QB was injured or limited. The backup, in this case Stidham, would have to run both the first-string offense and the scout-team offense, and it's difficult to do both well. That's why they have Kessler.
And have had him for most of the season, so I imagine this is a situation they envisaged -- Brady out or limited, need two more quarterbacks. I had been wondering why they had Kessler, when limited roster spots have led to them waiving key players who could have helped (and still might have been able to help if they had them). This is why.