People are shocked at Law's severed arm and titling him as the second version of Shanks.
Even though his arm got severed, you guys are forgetting that he has Ope Ope no Mi. He's a doctor. He can merge back his own arm with his skills.
Still, it's not that simple. He switches between his right and left hand depending on the technique he's using, or so it seems. When he cuts a person in half, he's holding his sword with his right hand. To put that person back together, he'd use both hands, using a special gesture to control both halves. To make two objects, or people, or both, switch places with each other, he uses his left hand. This is from what I've seen, at least.
So to put his arm back together, he'd need both his hands. Also, even if I'm wrong, you shouldn't compare his ability to a miracle cure. If it was that simple, the people on Punk Hazard would've been able to walk on their own two legs, instead of just borrowing a pair of legs from a horse or whatever. For all you know, that arm of his stays on the ground.