This may be true, but at the same time there's a difference between being in shape, and being in NFL tackling shape.
The Bills' D depends heavily on the CBs being strong, solid tacklers, and tackling is a little like boxing in some ways. No matter how good your strength and aerobic training is, you can't just step into the ring and expect to take a lot of shots from a professional boxer. Your body can't take it right away, it takes time to adjust to being hit.
I suspect the same is true for defensive players in the NFL. Tackling is hard on the body, especially on the knees when you're trying to stop a large, fast human from moving forward.
I agree with some who say if he's ready to come back, he should be gradually eased into it with a snap count.