There are basic questions to be asked about whether and how God would ever answer any prayer. If I discuss these, I’ll never get to the specific question about sport. So I’m just going to assume that God does answer some prayers. That’s a big assumption, but it’s legitimate in philosophy to make these, at least for argument’s sake, in order to make progress on smaller, more specific questions. One could also ask whether or how God’s knowledge of the future affects His decisions about how to answer prayer: good question, but again, it will have to wait.

Why would one think that God would care about something as trivial as the outcome of a football game? Well, it depends on what sort of God you have in mind. Aristotle’s God thinks only of Himself. He ignores the universe. So He ignores football. The Christian God, on the other hand, has His eye on even the sparrow. He is love, and loves all He has made (though He is less than thrilled with what we have made of it). So He cares about everything about His creatures. Nothing is too small to concern Him. 

So if Christians are correct, it makes perfect sense to think that God might lend a hand about football results. If I love you, I care at least a bit about some of what you care about, even if I think it’s silly. You collect postage stamps. I think it’s silly, but if I love you and receive a letter with a nice stamp, I will save it for you: I care about your collection because I care about you. There are exceptions, of course. My capacity to care is finite, so I may well not care about some things that matter to you even if I love you. And if I think that what you care about is not good, or not good for you, I’ll care about getting you not to care about it, if I really love you. But God’s capacity to care has no limit. So if God loves you, you care a lot about Manchester United’s winning, and He sees that this care is not bad for you and at least morally neutral, that gives Him reason to care about the match’s outcome, and to care about prayers concerning it.
“The Christian God, on the other hand, has His eye on even the sparrow. He is love, and loves all He has made (though He is less than thrilled with what we have made of it)”
If God does care about these, one wants to know how He would decide whose prayers to grant. Here we have to consider God’s aims, their legitimacy, and what He knows. Again, it matters what God you have in mind. Aristotle’s God has no aims in the world at all. The Christian God, on the other hand, aims at the salvation of all people. This is a legitimate aim. If something will make you infinitely happy, and I know this, love you and want you to have this great good, it is legitimate for me to try to give it to you. This is also a “trumping” aim. It is something so important, so vital to everyone’s interest, that God would have to make it His primary concern in dealing with people if He is truly rational and loving. It would be neither rational nor loving of me to care more about giving you a ticket to a football match than about making you infinitely happy. (I take it for granted that the ticket won’t do that.)
“If God loves you, you care a lot about Manchester United’s winning, and He sees that this care is not bad for you and at least morally neutral, that gives Him reason to care about the match’s outcome, and to care about prayers concerning it”
If that’s right, then God’s primary concern in answering prayers is to raise the chances of salvation of those who pray (and those who don’t). That will be how He decides, and given the legitimacy of His aim, it is a legitimate way to decide. God knows everything about us, and this gives Him the best possible grip on how possible answers to various combinations of prayers affect the probabilities of people’s salvation. He then acts according to His primary aim. He is love, and so loves us. Because He loves us, He wants what is best for us: salvation. Because He is perfectly rational, in dealing with us, He places more emphasis on what is best for us than on anything that is less good for us. So salvation is His primary concern, and He does in answer to prayer whatever best forwards the chances of salvation for the largest number- if He does anything at all.

One possibility, after all, is that prayers offset. Suppose that Manchester plays Chelsea, 1000 fans of each pray equally fervently for victory, God loves them all equally, their prayers are equally worthy, and the effect on salvation-chances for everyone offsets: if Manchester wins, certain people are a bit closer and others a bit further from salvation, while if Chelsea wins, the same numbers are closer and further, though just who is affected how differs. If God favors Manchester, He favors the salvation of some, while if He favors Chelsea, He favors the salvation of others. But He loves them all equally. He has no basis for playing favorites, and it might well be immoral to make an arbitrary choice that significantly disfavors the salvation of some when He has the option of doing nothing at all. If it would be immoral, God would do nothing rather than harm anyone’s chances. He would simply leave it up to Manchester and Chelsea. The more people disagree in their prayers, then, the greater the chance that none of them get what they pray for. But that does not imply that they get no answer. “No” is an answer. So is “I love you, but I also love him, and I won’t hurt either of you.”
“He has no basis for playing favorites, and it might well be immoral to make an arbitrary choice that significantly disfavors the salvation of some when He has the option of doing nothing at all”
Suppose that you pray for Chelsea and Chelsea win. You might think that your prayer was answered. But that supposes that your prayer was among God’s reasons for favouring Chelsea, and that He acted on those reasons. There are at least four ways this could be false. One, which we’ve just seen, is that prayers might offset. He might simply have left it up to Manchester and Chelsea. In this case, what you prayed for did occur, but God did not grant any prayer. He could also have done this even without offsetting prayers- as it were, said “no” by inaction. He could have said a more forceful “no,” in fact, if what He’d done in response to the overall balance of prayer was to tilt the field slightly toward Manchester, and Chelsea nonetheless triumphed. Or, finally, He might in fact have said “yes,” but not to you. Perhaps it was best for you that they lose, but better for too many others that they win. So be careful what you pray for: you just might get it, and that’s not necessarily a good thing.