But he was quick to point out his United side were closest to runaway leaders Manchester City last season, even if they finished 19 points adrift.
Mourinho warned United could face a "difficult" season if the club failed to sign new players, but he is insisting he will be content to work with the players in his squad in comments that may not reveal his true feelings after a disappointing few weeks of transfer business.
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.
Jurgen Klopp's Liverpool did reach they Champions League final, which they lost to Real Madrid, but Mourinho is still bristling over the perception of last season as a disappointment for United, who also lost the FA Cup final to Chelsea.
A man now on the books of MLS side D.C. United said: "I am sure Jose (Mourinho) has been getting the team right".
"My view is that first of all it is hard for me to believe that we finished second when I listen, when I read - not much but sometimes I do", the United boss said.
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.
Why Derby County can still sign players despite transfer deadline closing
In football you need time to work and players on the pitch to work and we didn't have that until now. It is hard for me to understand we finish second.
"Since nine years, every season - except the first when I arrived at Barcelona".
'You are capable of making people who win nothing and finish below us look like serial winners.
Asked if his tactics that were often criticised for being too negative last season are going to change, he said: 'The target is Leicester match.
'Its hard for me to believe we finished second.
"In Championship Manager you can just press a button", he stated.
"Of course it's hard to understand for people in football that Tottenham didn't sign or sell players, but sometimes in football you need to behave differently", he said in his press conference earlier today, as per The Mirror.