goon Players have been playing while fasting for ages. Not sure why Billy would be any different.
Over the last few years there are more muslims in the PL so there are bound to be some that don't fast on game days, and some that do fast. I'd expect someone who hasn't eaten or more importantly hasn't drunk anything to maybe not be at his peak performance levels. Some may choose not to be available on game day.
Though I think the positive thing is that because it's an away game in Manchester those who are travelling are given an exemption from fasting whilst travelling. So if we travel on the day of the game then he may not be fasting at all. So maybe the whole discussion will end up being a moo point. 😉