I was going to say aneurysm or thoracic outlet syndrome but most people haven't heard of that, but it is vascular. There are three causes typically. One is idiopathic, or no root cause found. That would be a problem since there is no real fix. The other two involve external compression. One is from the first rib, which requires rib removal. The other is from a muscular compression typically from an accessory scalene muscle. That's also fixable. Recovery depends on what they do and if there is any need to intervene on the subclavian artery or vein. Regardless, if this is the diagnosis, it is better news than a labrum tear, but he's likely out for most or all of the season