The keeper played seven times for United – once in the Premier League, five in the League Cup and once in the Champions League.
But after the legendary former manager retired in 2013, Amos never got a look-in under either David Moyes or Louis van Gaal.
Amos, now 30, is making his mark at Charlton – having played for ELEVEN different clubs.
And he told The Sun: “The big thing I've learned since leaving is there's a world outside Man United.
“I was going into proper men's football week in, week out. At United I had been getting a little taste of first-team football and then I'd be out.
“I'd sit and wait and wouldn't be allowed out on loan. It was difficult to get a flow of first-team games.
“So what I learned was the Saturday-Tuesday-Saturday grind of men's football."