Despite serving as back-up for David De Gea, the Argentine says he is fulfilling a boyhood dream of playing for the Red Devils.
"When you get the chance to play for the greatest club in the world, and the chance to wear this shirt, it's almost like fulfilling a dream - a dream coming true," the 32-year-old said prior to Thursday's Europa League clash with AZ Alkmaar.
"So it's something you're not going to turn down. When I got the chance to come to Manchester United, it wasn't that I accepted that I was only going to play just a few games. It's not your intention to be second choice and, when [Louis] van Gaal gave me the opportunity, I grabbed it.
"Here, within this club, both the staff and the players are one big family and we have goals: to fight hard every time we go out onto the field, to try to win trophies, to keep our fans happy.
"I've spent four fantastic years at this club," he added. "So much so that I want to stay here as long as I possibly can. If I was playing more regularly and featuring in more games, I would be the happiest goalkeeper and person in the world.
"But the fact that I'm still playing the odd game here and there, I'm still very happy with my situation because I'm at a great club which I want to stay at."