It's not just about his left hand. His right side, memory and overall mobility is also seriously impacted.I hope Keith can discard his mental model of what it means to be a pianist, and find a different way forward while rehabbing.
Apparently he's been trying to recover for almost 2 years, and it's just not working. He can't even dream about playing anywhere near the level he wants. That may mean that he's lost not only motor functions, but also musical ability as well.