The Chelsea boss has seen United spend £76million on, among others, Memphis Depay and Morgan Schneiderlin, while City have forked out £65m on Raheem Sterling, Fabien Delph and Co this summer.
The Special One, who has only spent £12m, said: "Without doubt they have spent to try and catch us. They won't, because we will get better."
Mourinho claimed Chelsea would buy a £50m player but do not need him at the moment.
"Two players - one a left-back - but I wait and see," said Mourinho. "We have the money. Chelsea have done fantastic business selling players.
"There is no doubt teams have spent to catch us. That is what happens.
"When you win the title once it is more difficult to win it twice. You win the title in a country and you are almost lonely.
"In the Premier League how many teams can react against the champions? It is the same in Germany and in France.
"People react. In the summer the first thing Manchester City did was go for Raheem Sterling. If City had been champions maybe they would not have reacted so aggressively.
"I told the players that if we are not buying we just have to get better. Do we need a couple of players to create some improvement?
"The direct result is that when you have opponents who are economically powerful, they want to react.
"There is a possibility there will be new players because the market is still open. We are going to get a defender for sure."
Mourinho goes up against old foe Manuel Pelligrini today when City host Chelsea.
The pair have had a frosty relationship.
"He does not have my number. I do not have his," he said. "Why should we? If I met him in London I would say hello and, 'What are you doing here?'
"But I love playing against top teams and City are a top team. We get fantastic results against top teams."
