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.
"I am not confident", Mourinho said.
A man now on the books of MLS side D.C. United said: "I am sure Jose (Mourinho) has been getting the team right". However, there was no indication that it was a deliberate tactic on his part or that he was being economical with the truth.
"Like any big club, they need some silverware - in his first season at Old Trafford, 2016-17, Mourinho won two trophies - the League Cup and Europa League - but he won nothing last season".
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"My view is that first of all is hard for me to believe we finished second", he added.
Darren Burgess Reveals What Unai Emery Most Wanted To Change At Arsenal
Instead, they will have to rely on Shane Long, Charlie Austin and Manolo Gabbiadini to lead their line into the new season. But Spurs and [Harry] Kane must make use of the current momentum they have before it collapses one day.
"Is hard for me to understand, to feel, to believe we finish second".
"Is hard for me to understand, to feel, to believe we finish second - which I keep saying, and it is sometimes hard to believe, that I won eight championships and three Premier League's but I keep feeling the second last season was one of my biggest achievements in the game".
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.
Mourinho had warned it would be a "difficult season" if United did not make any more acquisitions and it now looks like his side will miss out.
"So I hope I'm wrong, but I don't see them challenging for the league this year".
Fosu-Mensah is popular with United fans after breaking onto the scene at the same time as Marcus Rashford.
"The target is Leicester match and the tactics to be changed in Championship Manager you can just press a button", he said.