Simpson launched his pro career with United.
And he told Sky Sports: "He, for me, is world class. I think people are speaking right now but honestly I think for an English player as well, honestly he's world class and he's only going to get better and better.
“He's that good, honestly. Honestly, he's got it all.
“He's quick in his mind so I think he doesn't need to have blistering pace running past people because he's clever and he thinks before everyone else.
“He gets in them little pockets, gets into the right spaces, he's kind of got eyes in the back of his head.
“The way he receives the ball and turns and puts defenders on the back foot.
“So he doesn't have to be your typical 'I'm going to run past you and beat you' because his qualities are there with his vision and his passing."