It is Murder because of 2 reasons
1 - Causation, as soon as he punched the guy, he has started a chain of events The chain is said to be “Forged in Titanium” and very hard to break. Or Sing Qua Non – without which not. Or more eloquently put, the “But For Test."
But for the punch, the victim would not have fallen and hit his skull.
2 -
In Causation, there is also The Thin Skull Rule – For example, if I had hit you on the head hard with a textbook and died, but unbeknown to me, you have problem with clotting factors in the blood. I have to take my victim as I find him, in that scenario might not be murder (that goes with the mens rea), but the fact that I entire ignorant of the medical condition is irrelevant. It’s not entirely logical but the criminal law in its wisdom has decided that if we go around hitting people on the head, or shooting or stabbing or anything else in a violent nature, we must accept the responsibility that follows.
The boxer is a professional, he knew the limits of his punches, and yet he went for it.