Harvey's surgery will be to address his thoracic outlet syndrome. The condition can cause numbness in fingers and/or pain in shoulders. It stems from a compression of nerves or blood vessels.
The condition is a serious one, although Royals pitcher (and former Met) Chris Young was able to recover and return to the game. Other pitchers, like Jaime Garcia and Chris Carpenter underwent the procedure. Carpenter never pitched again the MLB, and there is a concern Harvey won't be the same pitcher he once was.
Harvey has struggled this season. Before being shut down, Harvey posted a 4-10 record with a 4.86 ERA and 1.46 WHIP. All of those numbers are career-worsts. Boras attributed those struggles to his thoracic outlet syndrome.