And it will take an offer in the region of £63million for them to consider selling De Jong at the end of the season.
United have signed five players from La Liga since Sir Alex Ferguson walked away in 2013.
Ander Herrera moved from Athletic Bilbao back in 2014, after Louis van Gaal took charge, and became a cult hero during his stint in England.
Angel Di Maria transferred from Real Madrid that same summer but, ultimately, failed to live up to expectations before moving to Paris Saint-Germain.
Victor Valdes was also recruited under Van Gaal, though fell out with his former Barcelona boss before being allowed to leave for free.
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