He played against United twice during the 2013/14 Champions League group stages at the age of 20, with Shakhtar drawing at home but losing 1-0 away.
Willian, who made nearly as many Premier League appearances (16) off the substitutes bench as he did starts (20) last season, remained coy when pushed on plans his own future. Fred has been a revelation for the Ukrainian side for quite some time and topped the transfer list on many top clubs all around Europe.
Fred has already participated in the promotional activities associated with the announcement ahead of joining back up with the Brazilian national squad.
Handed his debut by Dunga in 2014, Fred's global career was disrupted after testing positive for the diuretic hydrochlorothiazide during the 2015 Copa America.
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He is set to become United's first signing of the summer, with Mourinho also closing in on a deal for Porto full-back Diogo Dalot.