Mourinho has repeatedly expressed a desire to bring in additional signings throughout pre-season with a central defender top of his list.
However, despite links to numerous targets, United are yet to reinforce.
“The market closes today. It is time for me to stop thinking about the market,” he said.
“I am not confident. I have to focus on the players I have and the next few matches.”
United have so far signed three players this summer – Brazil midfielder Fred for £47m, Portuguese defender Diogo Dalot and third-choice goalkeeper Lee Grant.
But Mourinho has been keen on bolstering his squad further after they finished second last season, 19 points behind champions Manchester City.
Although there has been speculation about a deal for Tottenham’s Toby Alderweireld, there has been no offer from United while Leicester remain determined to retain England international Harry Maguire, who has four years left on his contract.
The Portuguese manager also had to contend with a series of injuries to senior players during pre-season.
Antonio Valencia and midfielders Nemanja Matic and Andre Herrera are among those absent for the Premier League opener against Leicester on Friday (20:00 BST).
But he is hopeful that some of his World Cup players could be available with Paul Pogba, Jesse Lingard, Ashley Young and Marouane Fellaini now back in training.
“The ones who played late in the World Cup trained Monday, Tuesday, Wednesday, today,” he added.
“They are not fit and ready but we have needs. Today I will speak with them to see how available they feel they are.”
On Thursday he said his side is “ready” for the new campaign, despite being without six players for Friday’s game against Leicester, including Ander Herrera, who was injured against Bayern Munich at the weekend.
He wanted another centre-half but it appears there was no truth to the rumour that in the absence of alternatives, a late move for Kurt Zouma was in the offing.