The Red Devils spent £150 million last summer on various big-name players - including Angel di Maria, Ander Herrera and Luke Shaw - and Mourinho expects something similar to happen prior to the 2015-16 campaign.
United remain on course to secure a top-four finish, which will mean a return to the Champions League following a one-season absence, but the Premier League giants will be keen to close the gap between themselves and champions-elect Chelsea - and that will involve spending money when the transfer window re-opens.
"Manchester United have a phenomenal squad, including the most expensive player in the history of the Premier League (Angel di Maria) and he is not playing,” Mourinho was quoted as saying by the London Evening Standard. “So you see the quality of the squad.
"They are a club where Financial Fair Play doesn't make a big difference and they will invest hugely again in the summer.”
Man United: Expected Ins and Outs
Manchester United have been strongly linked to several players in recent weeks, with various reports claiming Mats Hummels, Ilkay Gundogan and Nathaniel Clyne are all set to join the Red Devils within the forthcoming months.
Van Gaal is also likely to bring in another striker, with Radamel Falcao and Javier Hernandez both expected to leave Old Trafford.
Paris Saint-Germain striker Edinson Cavani is understood to be on Van Gaal’s radar, while Burnley’s Danny Ings is a cheaper alternative.
United may not spend huge money
Whether Manchester United end up spending the same vast amounts of money, however, seems doubtful.
The Red Devils, barring last Sunday’s unexpected 3-0 defeat to Everton, have been in fine form in recent weeks and don’t require an overhaul.
They need strengthening in key areas of the pitch, though, particularly at the back.
Rafael and Jonny Evans are both expected to leave United, who could replace the out-of-favour duo with Clyne and Hummels from Southampton and Borussia Dortmund, respectively.
Hummels’ Dortmund teammate Gundogan could also join the Red Devils; the Germany international only has 12 months left to run on his contract at Signal Iduna Park and is expected to follow manager Jurgen Klopp out of the club this summer.
Manchester United fans: which players should Louis van Gaal sign this summer? Have your say by leaving a comment in the box below!