Alexis Sanchez will be out for a "long time" after suffering an "aggressive muscular injury", according to Manchester United manager Jose Mourinho.

United announced on Thursday that the Chile forward had sustained a hamstring problem and, speaking ahead of Saturday's Premier League match against Southampton, Mourinho told reporters he expects top be without Sanchez for the foreseeable future.

"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 that the injury happens means I know he is going to be [out] for a long time," he said.

"It is not a little muscular injury that [means in] one week or 10 days the player is ready."

Centre-back Victor Lindelof is set to miss most of the pre-Christmas period with a muscular injury and Mourinho's stated Sanchez's setback is worse, with the ex-Arsenal man not expecting to feature again until 2019.

"For me, and also for Alexis, the feeling before the scans is that the grade is different than the Lindelof one and the Lindelof one is not an easy one," he added.

"Alexis has what I used to call an aggressive muscular injury."

Speaking to MUTV, Mourinho added: "He has an important injury, I don't want to risk by saying for how long he will be out but from the top of my experience I would say he is not playing this year."

Sanchez has struggled for his best form since moving to Old Trafford on a free transfer from Arsenal in January.

He has started five of 10 Premier League appearances this term, scoring once, having already been hindered by abdominal and groin complaints.

United entertain Arsenal next Wednesday, while the Champions League trip to Valencia and Premier League visit to Liverpool are games Sanchez will miss.

Mourinho's men begin their 2019 Premier League calendar with trips to Newcastle United on January 2 and Tottenham on January 13 either side of the third round of the FA Cup.