The pre-contract agreement, which will net Mourinho £16million-a-year, equivalent to £308,000-a-week, will see the Portuguese take over in the summer.
Mourinho is said to have been in discussions with Man United for the past six weeks.
Confidence in van Gaal at Old Trafford has reached a new low following the 2-1 defeat against Midtjylland on Thursday and the hierarchy at United have turned to Mourinho despite reservations over his character.
While Mourinho’s reputation has taken a hit following his sacking by Chelsea earlier this term, the fact remains that he is a proven manager with a track record of winning trophies throughout his career.
United are in serious danger of fading into mediocrity following a disastrous campaign and you get the sense that letting Mourinho go elsewhere would be a mistake, particularly with Manchester City bringing in Pep Guardiola next season.