But Mourinho, wary of a Paul Pogba-esque situation, pressed for a buy-back clause.
Pogba left United for Juventus in 2012 but the Red Devils had to pay £89million to bring him back to Old Trafford.
Since then, the Red Devils have ensured that, when they sell a player with potential, they insert a clause into the deal.
Memphis Depay has a similar clause.
Meanwhile, speaking before the game, Januzaj dismissed comparisons with England’s Jesse Lingard.
Adnan Januzaj: Belgium star struggled during his Manchester United days (Image: GETTY)
De Bruyne FLOORS Januzaj in Belgium's World Cup training
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“The World Cup is a big stage to show yourself, everyone is watching, so for me it will be very important," said Januzaj.
“In the Premier League I felt good, in my first year I made a difference in England. The second year was harder because I didn't have game time.
“If I had game time I would have grown up as a player too. Jesse Lingard was on loan a few times at United. Me, I started with a big bang there.
“I had a different way from Jesse. He had his way, I had my way. But I am still only 23 years old and I am happy with what I have achieved up until now.
“England will be a great game, with two big nations. At the minute we are the two favourites to go through. It will be a battle between us and England. But in football you never know, you always have to be ready."