Speaking ahead of Saturday's Premier League clash with Swansea City, however, the Portuguese has insisted that he is "ready to go without a fourth signing".
"Unless something happens that puts us in the market again. Obviously, my plans were four but I also told them to be cool, there is no pressure from me, do what you think is best for the club," Mourinho told reporters.
"We will be together again for another window in January and one next summer so there is no pressure at all from me. I am happy with the squad we have. I'm ready to go without a fourth signing."
United opened their 2017-18 campaign with a 4-0 victory over West Ham United last weekend.