It’s going to be tough to prove a first degree murder case, since it’s preposterous to think someone would plan a murder like this. Second degree is probably what they’ll threaten him with, and then they will plea down to manslaughter for a guilty charge.
The mitigating circumstance would be dependent on whether the perp punched the victim right there or had to run across the field to before he could hit the victim.
If it was an attack right there, it would be hard to prove first degree, because the accused could claim that he was provoked by the ref’s call. If he had to run across the field, that means he would have had to plan the attack, which would be a first degree.