Now on-loan at Barcelona, Rashford says the culture and atmosphere at United held him back in recent years.
But in response, former United captain Keane said on ITV: "Towards the end of his time at United, there was obviously issues. He was part of the problem with the environment, especially one of the most experienced players at United,' he said.
"He should have been setting the standards and showing new people at he club what it’s like to be a United player. But that’s been well documented. What we have to look at is he’s got this chance now with England and he deserved it.
"No doubt about his talent, he's obviously fantastic. But the bit that would have frustrated fans is his body language, not running back or not pressing properly. That’s the things he needs to tidy up and if he does there’s no reason he can’t become a world class player."