The bold option is to keep him and hope he regains form and proves he's worth what Juve paid for him.
The safe option is to take what money you can get for him just now in case he doesn't show any signs of regaining form and his value drops further.
I don't think I'd criticise either way, because I don't think there is a clear right and wrong thing to do here*.
*At least, that's how it appears from the outside. There may be longer term concerns about his physical condition that we can't be aware of. If that is the case, selling him while he still has a reasonable value is clearly the way to go.