The striker turned broadcast pundit believes Liverpool's mental edge over City following three victories against them last season - twice in the Champions League and once in the league - could be the difference if City do not runaway with the title in the manner they did last season.
Rivals Manchester City finished a huge 19 points clear of United last season.
Fred, Diogo Dalot and Lee Grant all arrived at United by early July, but nobody has followed despite Mourinho's explicit desire to bring in two more players.
"They return on Monday, trained Monday, Tuesday, Wednesday and today so you can imagine that they are not really fit and ready but we have needs", Mourinho said.
"It looked like I was saying something absolutely out of order, something insane, I repeat: Pre-season hard when you don't have your players to work, that's as simple as that".
Whatever the explanation, the lack of new faces compounds issues surrounding the United squad heading into the Premier League season.
"My view is that first of all is hard for me to believe we finished second", he added.
'You are capable of making people who win nothing and finish below us look like serial winners.
"Is hard for me to understand, to feel, to believe we finish second". The manager has to decide which way we play.
"But I keep saying and thinking and feeling that the second last season was one of my biggest achievements in the game".
Manchester United have been hit by injuries throughout the pre-season tour of the United States and there are plenty of first-team stars that have only just returned from their holidays due to exploits in the World Cup this summer.
'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'.
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