If we should keep him boils down to a cold and hard nosed assessment of probability.
1. What are the odds of him staying fit and having the season of his life?
2. What are the odds Moyes gets him fit and fits him into a system that suits the team, where he has a solid but unspectacular season?
3. What are the odds of him spending significant portions of the season injured, missing very passable chances when available, and generally frustrating. Maybe with the odd storming performance chucked in?
There's no cash out option here I am afraid.
10/20/70% for my money. We look stupid if 1. happens for West Ham or similar, but it really does not look likely given the past few years.