"I'm not confident", he said.
He had been hoping for more additions to a squad which finished 19 points adrift of Premier League champions Man City in 2017-18, but has seen his hands tied in the transfer market.
Premier League legend Alan Shearer has warned not to write off Manchester United's title chances, despite Jose Mourinho's despondent mood this summer.
"So it's time - at least for me - to stop thinking about the market because the market will be closed, so I will have to focus on the players I have in relation to this first couple of matches".
"I will have to focus on the players I have in relation to the first three matches, because after Spurs there is an worldwide break and I have a couple of weeks to recover people and prepare them better".
Mourinho sounded pessimistic that he would make any last-minute signings when talking to media on Thursday.
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.
"My view is that, first of all, is hard for me to believe we finished second", he said. In football you need time to work and players on the pitch to work and we didn't have that until now.
The Red Devils coach has cut a sullen figure throughout pre-season with the former Chelsea boss unable to hide his dissatisfaction with Brazil global midfielder Fred being his sides only high profile signing.
'You are capable of making people who win nothing and finish below us look like serial winners.
'I said during pre-season - it looked like I was saying something absolutely out of order, something insane - but I repeat: pre-season is hard when you don't have your players to work, it's as simple as that'.
Mourinho has won three Premier League titles with Chelsea and two Champions League trophies as Porto and Inter Milan manager.