Real Madrid have consistently been linked with a move for De Gea, but he has not let speculation affect his performances on the pitch.
After watching the Spanish stopper single-handedly keep Arsenal at bay in United's 3-1 win at the Emirates, Ducker told 'Sky Sports':
"People forget that United and Mourinho gave De Gea the option to go. Mourinho is on record saying he didn't want to keep players who wanted to go.
"But for various reasons Madrid withdrew from that and United kept De Gea last summer more by accident than design.
"What is remarkable about De Gea is that unlike Mesut Ozil and Alexis Sanchez, who have only just started performing in recent weeks, his head was turned but he has the ability to block things out and be the ultimate professional.
"This was evident when Louis van Gaal dropped him from the team for the first six games of the 2015/16 season, he came back in and was absolutely brilliant straight away.
"That takes a special player who has the mentality to detach themselves from what's going on in and around the scenes. We often look at players as robots but they are not, they are affected by things off the field as everyone is in everyday life. He is almost machine-like.
"People talk about title credentials and no team wins the league without an excellent goalkeeper. De Gea saves points, at least 10 a season.
"He is a remarkable goalkeeper and United's ability to keep him, despite Paul Pogba and other signings being good, has been the best piece of transfer business they've done."
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