While the Portuguese had already discussed these plans with executive vice-chairman Ed Woodward prior to Tuesday's Champions League last-16 elimination to Sevilla, that 2-1 defeat has only strengthened his resolve.
Mourinho has convinced his boss that the team needs further work, and more top-quality players are necessary to compete with Manchester City. Woodward is fully behind that.
Mourinho particularly wants to bring in a centre-half to partner Eric Bailly, with Barcelona's Samuel Umtiti and Tottenham Hotspur's Toby Alderweireld the main targets, as well as a controlling midfielder.
While most at United are now hopeful that the manager's relationship with Paul Pogba will be resolved after recent difficulties, it still isn't completely fixed, and intermediaries have been involved in devising contingency plans if it again worsens.
One plan was a potential swap for Paris Saint-Germain's Marco Verratti, especially since the two share Mino Raiola as agent, but that possibility is still seen as some way off. United still value Pogba as a player for their future, and all involved are determined to make it work.
There is also new uncertainty over the future of David De Gea. While United had been relaxed about his commitment to a new deal, his current contract runs out in the summer of 2019, albeit with an option to extend it by a year.
The club have been slow to discuss the situation in detail, and it is understood that the goalkeeper's camp are conscious of the contracts signed by Pogba and Alexis Sanchez, and that his importance should put him in line with them.
Real Madrid retain a strong interest in De Gea, and sources close to both Old Trafford and the Bernabeu feel it remains possible that the Spanish club will broach the possibility of a deal for Toni Kroos.
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