Asymmetrical loss of sharpness is usually due to decentering of one (or more) lens elements. Canon can certainly correct the problem, not sure of the cost (depends on the severity), but figure $200 more or less.+1.
You can test for decentering. Use a flat brick wall, make certain you are setup absolutely perpendicular to it. Rotate the camera at 90 degree increments, taking images at each. It the soft area rotates with the image, send it to Canon with a description of the issue.
Decentering happens as a lens is used, there can be various internal causes, usually related to bumping the lens. Canon will replace any damaged parts, and realign things. The cost may vary depending on the parts and labor required.
When buying a used lens, its a good idea to check for decentering since its so common.