I'm not talking about historical rivalries, but rather rivals when it comes to a title race or a top four finish. We loaned players to Liverpool(Moses) and Everton(Lukaku), because we didn't consider them a threat to our title ambitions. Had we known Liverpool would prove to be a genuine threat, we wouldn't be selling or loaning them anyone. Same goes for other clubs. Tottenham, for example, isn't a traditional rival to United, but that doesn't mean they'd welcome your approach for their best players. And so on.
And yes, if it's impossible to hold on to a wantaway player, all those clubs would prefer to sell them abroad rather than strengthen a domestic rival. But each player's situation is different. Since we're talking about Mata here, he's 25, a proven top quality player, great attitude,on sensible wages and is, by all accounts, have several big clubs as suitors across Europe. He's also under contract till 2018. The club is in a very strong position, when, and if, it'll come to selling him.