The pair were not named in the Red Devils squad as Mourinho’s men slumped to a disappointing defeat by the Magpies at St James’ Park to further dent their hopes of catching Manchester City.
Matt Ritchie scored the only goal of the game in the second half as the Red Devils lost their second Premier League game in three outings.
Mourinho confirmed after the game that both Herrera and Rashford were missing due to injury and said that the club are not sure when the duo will be available again.
Speaking at his post-match news conference on Sunday, Mourinho said: “Both [Herrera and Rashford] are injured.
“I don’t think so [they will be back soon], but let’s wait a little bit.
“When it’s little tweaks in muscles you never know until you know for sure, but it’s not possible for them to play today.”
Manchester United are back in action on Saturday afternoon when they take on Huddersfield Town in the fifth round of the FA Cup.
The Red Devils remain in second place in the Premier League table but they are now just two points ahead of third-placed Liverpool FC as the race for Champions League qualification hots up in the top flight.
Manchester United finished in sixth place and won both the Europa League and League Cup under Mourinho last season.
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