United paid Shakhtar Donetsk £52 million for the Brazil worldwide, but he has only started six Premier League matches.
The Red Devils have drawn their last two games 2-2 against Arsenal and Southampton and need to bounce back at home to Fulham tomorrow.
Mourinho will find out later on Friday who may be available for Fulham's visit on Saturday.
"There have been more rumours of Jose Mourinho leaving Manchester United". He has also been an unused replacement in five of the past seven games.
Mourinho said Fred was adapting "step by step" to life in the Premier League and added: "You have other players in other clubs that needed time (to adjust) and you have some of them that play even less than Fred is playing".
"I think Fred when the team is defensively stronger and doesn't need in midfield people that are more anxious in giving some balance to the team than being involved in creation and attacking dynamic, the day we are stronger defensively I think the horizons for Fred change completely".More news: Kevin Hart steps down from Oscars hosting gig, apologizes for homophobic tweets
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United, in eighth place in the Premier League, have already conceded 25 goals this season - compared with Liverpool's six - and have a goal difference of minus one.
"Jose Mourinho brings all his experience and knowledge of the game to us all, not just me", said the 19-year-old. "The spirit and fight is there and we will get there".
The United manager is full of respect for new Fulham manager Claudio Ranieri, the man he replaced when taking his first English job at Chelsea in 2004.
"And as footballers, we just enjoy our football and want to win games".
"The game is not going to be 11 against zero, it's going to be 11 against 11".
Arsenal are a good side, but they're due a downturn in results, this sort of thing won't happen all season.
"The points total sometimes doesn't say much about the real potential of the team".