Manchester United released a statement on Thursday, stating that they were investigating the severity of the problem Sanchez had suffered.
In an interview with journalist on Friday morning, Mourinho said: “No scans yet but the player is experienced, the player knows what the injury is.
“And also, from the top of my experience, the painful scream, and the way the injury happened, I know that it is going to be for a long time.
“It is not a little muscular injury that, in one week or 10 days, the player is ready.
“For me and also Alexis, the feeling before the scans is that the grade is different than the (Victor) Lindelof one, and the Lindelof one was not an easy one.
“So, I think Alexis has what I used to call an aggressive muscular injury.”