The Daily Mail says Fernandez is being closely watched by United this season.
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The 18 year-old centre-back is now training with Carlos Corberan's senior squad.
Fernandez's contract has a buyout clause of €4.5m, which can be triggered this summer. However, the option will double at the end of August.
United also are scouting Gambian defender Sedi Kinteh, of Tromsø. The teen is also being followed by Chelsea and Swiss side Young Boys.