United's buildup to the new campaign has been dominated by Mourinho's clear frustration over transfers, which has brought his relationship with executive vice-chairman Ed Woodward into the spotlight, AFP reported
The only arrivals at Old Trafford this summer have been Brazilian midfielder Fred, teenage fullback Diogo Dalot and backup goalkeeper Lee Grant.
In sharp contrast, the Champions League finalist Liverpool has spent more than £170 million ($220 million) on new signings including Brazil's World Cup goalkeeper Alisson and midfielders Fabinho and Naby Keita.
"My CEO knows what I want and I still have a few days to wait and see what happens," the Portuguese manager told MUTV ahead of his side's preseason friendly defeat by Bayern Munich on Sunday.
"The other clubs who compete with us are really strong and already have fantastic teams. Or they are investing massively like Liverpool, who are buying everything and everybody.
"If we don't make our team better it will be a difficult season for us."
United was beaten 1-0 by Bayern at the Allianz Arena, five days before its Premier League campaign begins at home to Leicester.
Mourinho, without some key players due to their involvement at the World Cup, including Paul Pogba, Romelu Lukaku and Jesse Lingard, brought on academy prospects James Garner, Axel Tuanzebe, Tahith Chong and Demetri Mitchell during the second half.
At fulltime, he compared his youthful team to the raft of talent at Bayern's disposal, including experienced players such as Robert Lewandowski and Arjen Robben.
"Finally, it's over. Finally, such a difficult preseason is over," Mourinho told mufc.com.
"The players were, once more, brilliant. They could not have done better than what they did. (It was a) fantastic experience for the boys."
Mourinho, who has been linked with moves for Tottenham defender Toby Alderweireld and Leicester center-back Harry Maguire before the transfer window shuts, selected Alexis Sanchez as captain in Bavaria.
Reports in Germany on Sunday said United has also expressed an interest in Bayern defender Jerome Boateng.