Now on-loan with Barcelona, Rashford scored twice last week in their Champions League win at Newcastle United.
"There's no doubt the talent that kid has got, it's ridiculous, he's shown up in a big game, and that second goal was brilliant," he said.
"I really struggled to feel happy for him, to feel pleased for him.
"Mainly because his attitude, his attitude at United. The time he was leaving, I thought it was just disgrace at United, not trying, basically.
"Look, if you fell out with a manager, then OK, but remember, you've got teammates you're playing with.
"You've got a massive 80,000 there, whatever it is, every single week that you have to put a bit of effort in.
"The amount of times I saw him at United walking because he wants to leave, I thought his whole demeanour was a disgrace.
"I think he actually quit on Manchester United. And once you quit once, you'll quit again."