Opinion of United fans is not a laughing matter at all. But using that as a base for argument, c'mon. The ratings are given based on people's opinion, which means the argument is subjective.
Now compare that to statistics (no, they're not everything), which are facts.
I can guarantee that if a United player would talk shite about United fans in public, and would continue his performances at the exact same level, his ratings would go down. Why? Because he played worse than before? No, because he was not as popular anymore.
Posters' preconceptions influence ratings, it's natural. And reading the comments on the Caf, Lingard is among the least popular players in the squad. Along with Fellaini.