"(There have been) no scans yet, but the player is experienced, the player knows what the injury is", Mourinho said.
"It's not a little muscular injury that in one week or 10 days, the player is ready".
"To win as many points as possible", Mourinho began.
"For me and also for Alexis, the feelings before the scans is that the grade is different than the [Victor] Lindelof one and the Lindelof one was not an easy one so Alexis has what I used to call an aggressive muscular injury".
If Mourinho is accurate with his forecast then Sanchez will miss up to 10 fixtures, including the visit to Old Trafford next week of his former club Arsenal.
Jose Mourinho has claimed Manchester United will need "almost a miracle" to finish in the top four this season and admitted Scott McTominay and Nemanja Matic "do not know how to play in defence" despite starting both midfielders there against Southampton.
Sitting eight points shy of fourth-placed Arsenal after 14-games of the current season, defeat on Wednesday night to Unai Emery's side would likely confirm the accuracy of Mourinho's sentiments.
"For us, it's a very exciting match. If we are stronger now, we need to [show that] on Wednesday".
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"I think the result is quite fair in spite of the fact we were more dominant in the game", Mourinho told BBC's Match of the Day.
"We want to be smelling these positions".
It seemed as though United would go on to win the game but their display in the second half was flat and uninspiring.
Manchester United goalkeeper David De Gea has turned down a deal worth £275,000 a week to stay at the club. Any decision from De Gea may have to wait to the summer. We have the best goalkeeper in the world.
Sky Sports News understands that Quique Sanches Flores has also been under consideration at St Mary's, while a string of managers such as David Moyes, Nigel Pearson and Carlos Carvalhal have also thrown their hats into the ring.
But the Spaniard is playing a waiting game and Mourinho believes his agent Jorge Mendes won't push him into signing a new contract.
"His agent lets him think and decide by himself, so I think the point is just to agree the contract that the club wants to give and the player wants to accept". They keep, keep going to complicated football.