The Spaniard is contracted to the club until the end of this season, although Man United have an option to extend his deal by another year.
Jose Mourinho noted recently that he expected De Gea, thought to be happy at Old Trafford, would soon commit his long term future to the club.
But according to Castles, there is no deal in place to keep De Gea, who believes he should be earning the same amount as Alexis Sanchez and Paul Pogba, who both receive around £250,000-a-week.
The keeper has good reason to be United’s highest earner having been voted the club’s best player in four out of the past five seasons, winning the ‘Golden Glove’ last year to crown a superb campaign.
World record transfers for keepers Allison Becker and Kepa Arrizabalaga over the summer, neither of which are anywhere near De Gea’s level, further bolster De Gea’s case that keepers of his quality need to be recognised.
United should not waste time with this. If they don’t offer De Gea the big money he deserves, some other club certainly will.