The defensive midfielder has been linked with the Premier League giants for some time, with rumours suggesting that David Moyes has had him scouted on a number of occasions over the course of the past few months.
It is now understood that United have settled on a fee with his employers, and that the 21-year-old will put pen to paper before the World Cup starts.
The figure to be paid will trigger Carvalho’s release clause, which was deemed to be necessary after the Portuguese club dug their heels in in a bid to keep the man being dubbed as their country’s top emerging talent.
Manchester City and Chelsea have also been linked with the Angola-born youngster, but it is thought that United’s desire to inject some physicality into their midfield has won them the race.
Moyes is keen to follow up the deal with a move for Bayern Munich’s Toni Kroos.
The German playmaker is stalling on signing a new contract at the Allianz Arena as he holds out for a significant pay rise.
United are believed to be willing to offer him the salary he desires, but they face competition from a host of big clubs.