We know exactly what's going through the coaches' minds. Sherfield was taken off the field with the primary starters like Davis and Kincaid and much of the starting OL. That tells you how highly they think of him. As of right now he is locked into what the Bills call WR4, the same role Davis had as a rookie. He'll split time in the slot and outside and he'll be asked to block quite frequently.
Isabella will have a completely different role from Sherfield if he makes the team. I have no clue why you think their roster spots are connected in any way.
They're pumping up his trade value by taking him off the field after two series in which he got one stupid target? You're on your own with this one, man.