Since the 2008-09 season when Alex Ferguson's United defended their crown, the reigning champions have stumbled at the first hurdle and failed to retain the title.
"I'm not confident", Mourinho said, when asked whether there would be more players arriving before the deadline.
I've spoken to Pep twice on Sky since the end of the season, we did a programme the other day with Dave Jones and Jamie Carragher asking questions.
The Merseyside outfit have signed AS Monaco midfielder Fabinho, RB Leipzig star Naby Keita, Stoke City playmaker Xherdan Shaqiri and AS Roma goalkeeper Alisson Becker in the summer transfer window so far.
Sources also say United have baulked at the agent fees involved in signing Mina from Barcelona while sources close to the club have knocked down reports of a bid for Chelsea's Kurt Zouma.
In 2012-13, City watched United out pace them by 11 points as they were forced to settle for second, the year after winning the title on goal differential on another dramatic final kick of the season.
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Sergio Romero, Valencia, Marcos Rojo, Dalot, Ander Herrera and Matic are all injured for Leicester's visit, while Mourinho needs to check on the fitness of those that returned late from the World Cup. "You can not expect them to be fit". Is not an opportunity, is a match, an important match, we would prefer to have a couple more days but decision was to be first to play and that's it.
"It is hard to believe that we finished second because you are capable of making people that finish second look like they were relegated, and people who win nothing, finish below us, and you make them look like serial winners. I keep saying that I won eight championships and three Premier Leagues but I keep feeling that second last season was one of my biggest achievements in the game".