The 28-year-old made his first Premier League start of the campaign in the 2-2 draw at Stoke City on Saturday but was left out of the matchday squad yesterday as Mourinho instead opted to play Nemanja Matic and Paul Pogba in the midfield.
"No reasons to be out," Mourinho told reporters last night. "We are playing only with two midfield players, with Matic and Pogba, who are playing really well and without reason to change.
"The only match we decided to play with three [Herrera] played, today we went back to two again and I had on the bench [Michael] Carrick and [Marouane] Fellaini, which were also important players last season.
"We have a group of players who must be ready to play and not to play. Ashley Young was not on the bench in the last match and today he started the match and played 90 minutes and the next match you can have another surprise of some player you think is going to play and is not going to play.
"It's the squad we have, I have to use the players and that's the life in clubs with squads of quality."
United are back in league action this Sunday when they welcome Everton to Old Trafford.