If you can locate the center point of the joists where the subfloor is attached, you can cut the plywood out parallel to the joists along the center line of the joists. Ideally you want any repair to span three joists.
To do the job, you need a circular saw set at the depth of the subfloor (15/32 - 3/4" normally and up to 1"). The idea is to cut along the centerline of the joists and remove a rectangle of material. Don't try to remove the entire 4/8' sheet. It is glued and nailed along the edges and is usually tongue and groove. You may be able to get away with removing a section as small as 31" x 12" depending on the area of rot you are dealing with.