1. Is this even remotely plausible? I'm pretty sure there's no way that someone could stop a full-speed sword slash without kissing goodbye to their hand, but if the sword is at a point where it doesn't have much momentum - say, the swing's been completed and the character reaches out and shoves it away before it can reverse direction - is this a conceivable scenario?
2. What's going to happen to his hand (right away)? It seems likely that he would catch the majority of the damage on the outside/pinkie side of his palm. I think I'm correct in saying that this would cause some serious problems for the ulnar nerve and the fourth/fifth fingers of the hand, but what kind of problems and exactly how serious? Will he be able to feel anything at all? Will he be able to move it? What about his other fingers? What about his wrist and arm? How quickly (if at all) is there going to be a major bloodloss/shock problem? Are bones likely to break, or are we mainly just talking slashing damage?
3. What's going to happen long-term? Hands are fiddly things - I know it will never be fully functional after that sort of injury, but exactly how non-functional is it likely to be? Will he be able to pick things up? Bend his fingers? If he loses feeling, will feeling come back a little, a lot, or not at all? How long would that sort of thing take to heal - both the serious deep-tissue and nerve damage and the more superficial skin-starting-to-knit-back-together sort of healing? Are there other complications that I might be overlooking?
I know infection would be a major, major concern, but I feel pretty confident in my ability to get that right. It's the physical damage itself that I'm not so sure about.