The Germany international was a part of his country’s successful World Cup campaign last year after emerging as one of Europe’s top young centre-backs after leaving Everton in 2012.
He initially joined Sampdoria, but his performances swiftly earned him a transfer to Valencia, whom he’s shone for in La Liga action.
And his showings have attracted attention from United, with reports claiming a 2016 move is on the club’s agenda.
Los Che are eager to keep their man, but offers of £22m may too good to refuse even though his release clause stands at £37m.
Bayern Munich are also thought to be keen.
Mustafi only made one senior appearance for Everton, as a substitute in a Europa League defeat at the hands of BATE Borisov in 2009.