Can you drop one night of soccer on days you're feeling particularly drained? Or maybe just play half a game? As your training intensity increases at boxing you don't want to be so drained it effects your performance. How are your sleeping patterns? Of course you'll want to taper leading up to fight night but I've found when I have a longish build up to a figth or several fights in close succession there's no substitute for rest. Although I'm not a nutrition guru so I'll still leave that for discussion with those who are :-)