The Red Devils have contended with regular speculation linking the Spanish shot stopper with a move to Real Madrid.
De Gea was on the brink of a move to Real Madrid in the 2015 summer transfer window but a deal fell through at the last moment.
Manchester United tied De Gea down to a new deal to ensure his long-term future at the Old Trafford outfit.
The 20-time English champions managed to keep De Gea last summer despite a fresh wave of speculation.
De Gea isn’t the best paid played at Manchester United despite being their top performance in recent seasons.
Indeed, Alexis Sanchez and Paul Pogba earn more than the Manchester United number one goalkeeper.
Reporter Castles believes Manchester United won’t sell De Gea in the 2018 summer transfer window.
“Man United and Mourinho will resist,” Castles told the Transfer Window podcast.
“I expect Madrid to target their resources elsewhere. He stays one more season at least at Man United.”
Manchester United finished in second place in the Premier League table last season to secure their spot in the Champions League.
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