Mourinho has already said he has a good depth to his squad, with Herrera being one of the players he may have to call upon with Pogba sidelined for a possible eight games.
Tuesday's long-awaited Champions League return came at a cost, with the 24-year-old only lasting 19 minutes of the 3-0 victory against Basle.
When asked how long he expected to be without Pogba, Mourinho replied: "I don't know".
"After 2-0 I think everything changed and we stopped to play, stopped to think, stopped to play seriously, ' Mourinho said, as quoted by the Daily Mail".
"So, for me, it's just simple and objective: he's not playing this weekend".
The France worldwide left Old Trafford on crutches and Mourinho is unsure about his absence beyond Sunday's Premier League clash with Everton.
Expanding later in the media conference, Mourinho continued: "We have players and we have players waiting for an opportunity".
"I don't know [the extent]".
"I don't know. I don't know that I just it is a muscular injury", he told reporters. We have previously lost players during crucial moments. Pogba has struck up a friendship with Lemar and has sent him Instagram messages suggesting he is brokering a deal to bring Lemar to Old Trafford.
They report that Herrera "upset" the manager at the end of last season after he "innocently invited" a friend to the club's training ground who has "close links" to Celta Vigo - United's Europa League semi-final opponents.
The Portuguese also added that Marcos Rojo's rehabilitation from knee surgery has progressed as scheduled, but he is still not back in full training and will only make a complete recovery in the next few months.