Mourinho spent nearly £150 million on three new players this summer after splashing out the same amount on four new arrivals 12 months ago.
He made no secret of his desire to make one more signing before last week's transfer deadline, although interest in Ivan Perisic, Gareth Bale and Danny Rose ultimately came to nothing.
United have made a flying start to the season with three wins in a row, 10 goals scored and none conceded, but Mourinho says he will not be truly satisfied with his squad until further signings arrive next summer.
"I have said three [summer] transfer windows is what I need to have the team I want to have -- the team [that] I think can bring Manchester United to the top of English football and to close the gap to the top of European football again -- and I still think we need that third transfer window, but I'm not going to wait for the third transfer window to try to reach it," he said.
Jose Mourinho said he will aim high this season but feels Manchester United still need to invest more.
Despite missing out on a fourth summer signing, Mourinho says he is pleased with the business executive vice-chairman Ed Woodward and the board did manage to do.
The £75m spent on Romelu Lukaku and the £35m paid to Chelsea for Nemanja Matic already look like good deals after impressive starts to the season.
Mourinho says his club did well to secure Lukaku, Matic and Victor Lindelof before Neymar's world-record move €222m from Barcelona to Paris Saint-Germain turned the transfer market on its head.
He says United's quick business saved them nearly £75m on Lukaku alone, explaining: "I think we were very clever.
"We thought that something could happen that could change the market forever. Normally the last part of the market is less expensive, but this season, after Neymar, everything changed -- and changed for the worse in terms of prices.
"I think Lukaku on Aug. 31 would have been £150m. Matic would have been £60m or £70m. Neymar changed everything."