Ronaldo got booed at Real Madrid - as supporters we need to create an environment where these players (that are paid millions) don't rest on their laurels, cheering them when they're playing crap is one of the many reasons why we're stuck with overpriced and overpaid players.
If these guys can't handle being booed when they're in poor form, then they need to leave, it should really be that simple.
Ideally weak minded players shouldn't be purchased in the first place so the situation where fans boo players don't happen often, sadly our team is full of players that are, and have been playing subpar for years.
Put it simply they've been coddled into this false sense of entitlement where they assume supporters of Manchester United should automatically support them, no matter what, and its attitudes like yours where you're wagging your finger at other supporters for pointing out the obvious that creates this environment.
Obviously one off bad games won't make supporters boo them, but I think many fans have just become fed up of this lot and its difficult to cheer them when it doesn't seem to make a difference.