Mourinho has seen United fall 15 points behind runaway leaders Manchester City in the Premier League title race, as well as suffer an embarrassing League Cup exit against Championship side Bristol City.
Successive 2-2 draws in the league, against Leicester City and Burnley, have only deepened the United manager’s frustration.
Mourinho accused his players of “childish” decisions as they conceded a late equaliser at Leicester last Saturday, and was every bit as angry with the defending against Burnley on Boxing Day.
The quality of United’s defending has become an increasing problem for Mourinho of late.
Having kept eight clean sheets in their opening 10 league matches, they have managed just two in the past 10.
Injuries to Eric Bailly and Phil Jones at various stages of the campaign have not helped in that respect, but with the Ivory Coast international set to be out for another couple of months as he recovers from ankle surgery, Mourinho has to find a solution at the back quickly.
Chris Smalling, another of United’s central defenders, missed the Burnley game with a groin injury picked up at Leicester, and the manager still seems reluctant to use Victor Lindelof in that role in Premier League games.
Mourinho is still looking for his best combination in attack too.
Romelu Lukaku is struggling to rediscover his early-season form, while Zlatan Ibrahimovic is only beginning to find his way back to match fitness after seven months out with cruciate knee ligament damage.
France striker Anthony Martial missed the Burnley match with a knee injury, and will be assessed before a decision is taken on whether to include him in the squad to face Southampton.
Jesse Lingard has played himself into contention for a start after coming on as a half-time substitute for Ibrahimovic to score the two goals that earned United a Boxing Day point.
“We always have that fighting spirit, that never-give-up attitude and we’ll see where it gets us,” Lingard said.
– Extra motivation –
Paul Pogba, meanwhile, is set to continue as captain, having taken over the role from the injured Antonio Valencia.
“Against Leicester we felt that it could be an extra motivation for him (Pogba) and that was it, there was no special decision,” Mourinho said.
“But maybe, because he’s a young guy and made in the club academy, he has the conditions to be captain in the future.”
Southampton have been sleepwalking their way into the relegation picture thanks to just one win in their last 11 games and will kick off at Old Trafford on the back of a 5-2 Wembley defeat by Tottenham on Boxing Day.
Saints have just sold Virgil van Dijk to Liverpool for £75 million ($100 million), a world-record fee for a defender, and under-pressure manager Mauricio Pellegrino is hopeful he will be given the funds to bring in at least three new players over January, including a direct replacement.
“At Southampton now it is impossible to bring in a player for £50 million,” he said.
“But maybe for us it can mean in the future one or two or three players with this money. If we reinvest it could be positive.
“Now the Virgil deal is done, my best wishes for him, but it has to be a positive impact to reinvest, a good decision for the future.”
Pellegrino revealed he had been studying United’s performance against Burnley in the hope of pulling off a similar success.
“I was analysing that game but now our target is to bounce back in the Southampton way, to play with more character and be more clinical,” he said.
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