Rashford has been at the club and the star boy during a period where we have sunk from being in the fight for real trophies every year, to regressing into midtable garbage. The player has shown glimpses of brilliance, which landed him hefty pay increases, but then he suffers multi-season long dips of form that produce some of the most abject performances we've seen from anyone thats ever worn our kit.
There's no question the physical ability is there. The problem is that the drive, desire, and the intangible spark that fuels winners to expect nothing less than victory, is completely and utterly missing in Marcus Rashford. What's worse, is that he carries himself as if he was on the 98-99 squad or is Cantona himself. He can't fix any of his shortcomings and dips in performance if he thinks he's already the man, and if he sees any criticism towards him as unwarranted and based on some other nefarious motivation not football related.
TLDR - Rashford is a headcase and loser and we're not winning anything with him at the club as the focal point of our attack.