I don't know how he can get hurt by knife when he is so much better at playing with it....

It makes sense, because he pulled the knife out of nowhere. He didn't expect it, since he had already laid him. Also, he's a bodyguard and I like to think that working as a bodyguard, surrounded by bodyguards, communicating a lot with bodyguards, even those of other people in the past, has made him think of them as a certain type of professional. He wouldn't expect a move that a thug would pull. But that's just an extra thought, he mainly had him down already and then he pulled a knife on him. It's thanks to his relfexes that he didn't get him in a more problematic way.