I would perceive something sketchy in the team which is left untold. Secondly the fan base of Manchester United is so toxic, they only praise and give accolades to prime players and when this players in question experience fatigue and flop, the lashed at the first sight they see flopping performance, they give absolutely no room for redemption or a second chance.
To be fair, isn't that every fan base, though? I think the toxicity is more than just lashing out at underperforming players. On top of that, the drop of performance isn't just one or two bad games, but consisten,t terrible games and so on. But yeah, I agree that players should just ignore noise from social media and focus on what their coach wants from them. At the end of the day, fans will cheer if you play great and will turn hostile whenever you no longer perform at your best. They're not your family so that's that.