I am going to argue on the positive side for the unfortunate Mets' pitcher. He has not yet decided between surgery and a nerve-block injection to treat the syndrome. Pitchers have been known to recover and return to pitch after being treated. Even if he is unable that does not mean his career relative to baseball is done; he has many baseball-oriented options.
Modern medicine means that recovering from a serious disorder like Thoracic Outlet Syndrome does not have to end an athlete's career. He has access to the best doctors and the best treatment, so there is a good chance that he will be able to come back and play baseball next season.
Thoracic Outlet Syndrome can be surgically treated but like any surgery it will take a lot of time to heal. For Matt Harvey's case, doctors are suggesting a removal of one of his ribs as a short term solution of keeping him in the game before the seasons end where a major surgery will take place.
Thoracic outlet syndrome is caused by the compression of nerves, blood vessels or both; we know that. We also know that his managers said he will be having surgery soon to correct the dilemma. Thoracic outlet syndrome, unless severe, will not cause life long damage when it is corrected. He and his managers have also commented that following the surgery he will be working very hard to get to the 2017 season.