It’s important to remember that your sylveon’s health is more important than his performance, anonymous.
That having been said, I would recommend getting a second opinion as quickly as possible. Modern medicine has reduced our need for amputation considerably; it’s only ever used as a last resort (namely, if the limb is completely unsalvagable due to extreme damage). Most infections from minor injuries can easily be handled with antibiotics.
However, you should act quickly, and if amputation really is your only option, put your sylveon’s well being first. With the right physical therapy and training, you can work with a disability and figure out how to help your sylveon succeed despite his amputated ribbon. At the risk of putting it too bluntly, you can’t do the same if he’s dead.