The Manchester United forward sustained the injury in a training session on Thursday and is set for up to a month out of action.
Manchester United star Alexis Sanchez is set for a spell on the sidelines after it was confirmed he has suffered a hamstring injury.
The Chilean has started just seven matches so far this season in a campaign that has been plagued by injuries.
Reports suggest Sanchez could face anywhere between two to four weeks on the sidelines, which means he is likely to miss next week’s clash against his former side Arsenal.
Having missed out on a place in the matchday squad for United’s win against Young Boys on Tuesday, Sanchez was expected to be required again during the upcoming festive schedule.
Mourinho was quizzed on the omission of Sanchez during the victory over Young Boys earlier this week, where he confirmed that the former Barcelona and Arsenal forward is not injured.
“No he is not injured, but it is quite curious because you always speak about the ones who are not playing,” the United boss said following the match.
“Today (on Tuesday) you had (Marcus) Rashford as a 9, that’s what you want, what you ask all the time, it’s what you had.”
Sanchez has just one goal to his name so far this season having lost his place in the side during the early stages of the campaign.
There has been speculation that the 29-year-old is unhappy with life in Manchester having joined the Red Devils from Arsenal in January.
The form of fellow United forward Anthony Martial will have played a part in Sanchez’s lack of opportunities in recent weeks, with the Frenchman netting six league goals since the start of October.
Altogether, Sanchez has managed just four goals in 30 appearances for the Red Devils, having notched 80 in 166 for the Gunners.