The Spanish stopper has failed to cement the No.1 jersey at Old Trafford since his big money move from Atletico Madrid in the summer of 2011 amid pressure from Anders Lindegaard.
De Gea's career at United has seen a spate of handling errors coupled with a string of brilliant saves, however his failure to settle in the Premier League will not be the precursor to his departure back to his homeland.
"I am happy at United and not thinking about a return to Spain," De Gea told The Sun.
"I've accepted the challenge to learn and improve as a professional. And fighting for my place gives me a bigger incentive to play well."
Brought to fill the shoes of Edwin van der Sar following his retirement, De Gea has endured a shaky first 15 months to his United career.
After begining his debut season as first choice between the sticks for United, a series of errors, including against Blackburn Rovers on New Year's Eve 2011, forced Sir Alex Ferguson to rotate both De Gea and Linegaard for the remainder of the campaign.
The Scot has continued that policy this term, however injury to the Denmark goalkeeper has handed De Gea the chance to start three successive league games.
Reports had suggested that coupled with De Gea's inability to settle in Manchester, his girlfriend, Spanish pop star Edurne García Almagro continues to travel between Madrid and the north-west.
Jorge Mendes, De Gea's agent, is understood to have been notified of the keeper's discontent at Old Trafford, which has sparked Madrid's reported interest.
However, while Casillas has served Madrid for 13 years, at only 31 the Spanish World Cup winner has enjoyed one of the best years in his career in 2012 and is contention for the annual Ballon d'Or award along with Barcelona's Lionel Messi and teammate Cristiano Ronaldo.
Having claimed the La Liga title, Casillas captained Spain to a third successive major tournament victory at Euro 2012, conceding just one goal, to Italy's Antonio di Natalie, throughout the event in Poland and Ukraine.
Therefore, while De Gea would initially be brought as a backup for Casillas, it appears likely the Madrid skipper will continue to play a prominent role at the Bernabeu, restricting the opportunities for development of the current United stopper.
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