"A further announcement will be made in due course".
United boss Jose Mourinho is looking to strengthen his midfield following the retirement of Michael Carrick and the uncertainty over the future of Marouane Fellaini at Old Trafford.
"The World Cup is important, for us to deliver a good performance, for the team to do well and for things to go as well as possible".
Fred was born in Belo Horizonte and moved to Europe in 2013 from Brazilian club Internacional.More news: Kim says committed to denuclearisation of Korean peninsula
With United aiming to close the gap with arch-rivals and Premier League champions Manchester City, Jose Mourinho has acted fast by bringing in the talented 25-year-old.
"He is excited to start but now is the time to think about the World Cup".
Brazil superstar Neymar was seen congratulating Manchester United's new signing Fred during the national side's practice session.
De Gea was once again voted as United's Player of the Season as they finished runners up to Manchester City. Fred is an all action box to box midfielder, who in theory should significantly improve United's style of play. He's played five years for Shakhtar, but still didn't become their top player.
"It's very hard, I don't want to be emotional but I have to be", the Portuguese coach told RT.
But I'm glad that I could play the last four matches.