Replacing a larger chunk of floor is not much more work, and in fact may end up being less work, than trying to piece in a very small patch.
One technique is to add some sister joists to attach a patch piece to. If you don't do this, then you have to get the old subfloor cut right on the centerline of a joist so a patch can share. This is tedious. Better to cut out a little more subfloor, and use the sister joist technique.
Sheet flooring requires an absolutely smooth substrate. You will have to either go over the entire floor with some 1/8" or 1/4" luan, or carefully feather all subfloor seams with leveling compound. Probabaly both.