Fred arrived at Old Trafford as Mourinhos marquee signing of the summer transfer window but the £60million midfielder has only managed six starts in the English top flight this season.
The Brazilian was dropped altogether for Uniteds win over Juventus in the Champions League and was an unused substitute for Sundays 3-1 defeat to Manchester City in the derby.
On Freds start at United, club legend Irwin told MUTV: The manager, like he did with [Victor] Lindelof, he is letting Fred settle and get used to the pace of the Premier League.
He came across from Shakhtar and I cant imagine the pace across there is anywhere near our league.
I think youll see the best of Fred in the coming few months. He is aggressive, runs around and is energetic.
I think well see more of Fred, I really do.
Fred admitted earlier this season that he has struggled to adapt to the Premier League following his move from Shakhtar Donetsk.
Im still adapting to many things and in the previous games I wasnt playing as well as I was playing before I joined the club, he said at the end of September.
Now I feel much better and to be able to score my first goal was really amazing.