Alexis Sanchez was absent after suffering a hamstring injury in training, which will keep the Chilean out of action until 2019.
So I think Alexis has what I used to call an aggressive muscular injury.
But he may not feature against Southampton on Saturday, and could also be ruled out of Wednesday's visit of former club Arsenal.
Sanchez is yet to have a scan to determine the extent of the damage but Mourinho said he was "screaming" in pain.
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The news will come as another blow to Jose Mourinho after he lost Victor Lindelof for around a month due to a groin injury last week but it's a real setback for Sanchez as he continues to endure a torrid time at Old Trafford.
"It is not the little muscular injury that in one week or 10 days the player is ready".
Manchester United's stars arrived at Carrington for training on Friday ahead of facing Southampton - with one notable exception.
Mourinho's team have conceded five goals in the opening 15 minutes of games this season, a joint high with Fulham and Cardiff, and the Portuguese coach has asked for a fast start at St Mary's. We need to improve our position in the table, we want to close the gap to the top four.
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However, on his first outing as the first team boss, Santiago Solari's Real Madrid side suffered a 3-0 defeat. Real are sixth in the table and six points behind surprise leaders Sevilla in an unpredictable season so far.
"But everybody for different reasons needs points".
'So we are going to have a very hard match against a team that I know are not going to have any problems; the team, the squad is good, the club is very well organised for many years, they are not going to stay in that part of the table for a long time, so it is a hard match for us'.
Mourinho was given a boost after United triggered a one-year extension on influential goalkeeper David De Gea's contract.
But United warded off the potential interest of several of their domestic and European rivals, much to the delight of Mourinho, who anticipates De Gea putting pen to paper on a longer-term deal soon. Manager wants him to stay. "His agent will let him think and decide himself. My feeling is that it will finish well".
Multiple Shot In Black Friday Shooting
The police officers involved in the shooting have been placed on administrative leave, pending an investigation into the shooting. We are working closely with the Hoover Police Department and are grateful for their swift action to contain the situation.