The 25-year-old will cost the English giants a reported fee of about £52 million ($69 million) and give Mourinho a powerful-looking midfield, with the Brazilian set to slot in alongside Paul Pogba and Nemanja Matic.
Born in Belo Horizonte, Fred moved to Europe in 2013 from Brazilian club Internacional and enjoyed success during five years in Ukraine.
Former Shakhtar Donetsk boss Mircea Lucescu has confirmed Fred has completed his transfer to Manchester United.
Fred has reportedly passed a medical and agreed terms on a move to Old Trafford on Monday.More news: U.S. teens are abandoning Facebook for YouTube, Snapchat
"A further announcement will be made in due course". He has a lot of qualities.
Manchester United's doctors are being kept busy this week as Diego Dalot is poised to arrive for his medical ahead of signing from Porto, according to Sky Sports. He goes to one of the best teams in the world. "If I was a club manager, I would be looking to sign him".
'He is very happy to come to the Premier League to play.
"He's excited to start but now it's time to think about the World Cup and think about the national team".
He recently appeared as an 82nd-minute substitute for his nation's 2-0 victory over Croatia last weekend at Anfield.
The 29-year-old, who has scored eight times in 55 worldwide appearances, felt last season was a good one personally but was coy about his plans after the summer.