Varela was former manager David Moyes's first signing for United in 2013 but made only four league appearances.
He spent his second season on loan at Real Madrid Castilla, the Spanish side's reserve team, and last season was loaned to Bundesliga club Eintracht Frankfurt.
They suspended him in May after he had an unauthorised tattoo, which became infected.
(Reporting by Steve Tongue; Editing by Toby Davis)
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