Commenting further, Mourinho admitted to being torn over who he wants to progress deep into the tournament - due to wanting his Red Devils stars to have a rest before the new season.
Neymar and several other members of Brazil's squad were seen congratulating Fred on the move at Tottenham's training ground as Tite's men prepared for their World Cup warm-up match with Austria.
The Premier League campaign will kick off on Saturday, August 11.
Manchester United have confirmed on their official website that midfielder Fred will be at Old Trafford next season.
United are in need of midfield reinforcements with Michael Carrick retiring and Marouane Fellaini out of contract.
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United fans are unlikely to get much of a glimpse of Fred at the World Cup in Russian Federation, though, as he is yet to start an global under current coach Tite.
Fred has played 156 times for Shakhtar since joining them from Brazilian side Internacional in 2013, and has appeared in the UEFA Champions League regularly.
Fred was handed a two-year ban between July 2015 and July 2017 after he tested positive for the diuretic hydrochlorothiazide during Copa America. Fred is a supremely talented player that is an all-round central midfield lynchpin that will add more solidity and mobility to a midfield that sometimes struggled with teams that sat back in huge numbers.
Fred did not return to the national team until a 3-0 friendly win over Russian Federation in March and Tite was keen for his transfer to be completed before the World Cup.