Manchester United striker Romelu Lukaku needed a rest - but not like this... Marcus Rashford struggled in his absence against Southampton

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When Jose Mourinho said that Romelu Lukaku needed a rest, this is not what the Manchester United manager had in mind.

Barely eight minutes had passed at Old Trafford when the big Belgian collapsed to the turf following a clash of heads with Southampton’s Wesley Hoedt. A further five minutes elapsed while Lukaku was treated on the pitch and then carried off wearing an oxygen mask.

Even if, as we would hope, he makes a speedy recovery, the United striker is unlikely to play at his old club Everton on Monday. That breather Mourinho spoke about will be an enforced one with concussion bringing a mandatory six-day layoff.

Romelu Lukaku of Manchester United lies injured during the match against Southampton

Lukaku suffered a head injury against Southampton at Old Trafford on Saturday evening 

It caps a frustrating time for Lukaku in which the early-season goals have dried up. He has become overworked and some have even said overweight as a result of bulking up too much since a £75million summer move from Goodison Park.

It’s worth noting that he missed the best chance of the first half before departing in a stretcher, heading wide from point-blank range in the fourth minute from Juan Mata’s excellent cross.

In recent weeks, Zlatan Ibrahimovic would have been the obvious replacement after returning from a career-threatening knee surgery.

Lukaku's worried team-mates immediately call for medical assistance and a stretcher 

But the Swede was not even on the bench against Southampton, omitted from the squad completely after being substituted at half-time against Burnley on Boxing Day.

It meant that Mourinho was forced to use a system – and personnel – he has been reluctant to adopt since his arrival at Old Trafford; one that does not involve a big target man and relies more on pace and movement.

 The Belgium international was hurt in a clash of heads with Saints' Wesley Hoedt early on

Mourinho has always favoured a striker in the Didier Drogba mould. He has cast doubt on the ability of Marcus Rashford or Anthony Martial to lead the line despite their progress under his predecessor Louis van Gaal.

Rashford was stripped and ready to go long before Lukaku was lifted from the turf here. He took up his more preferred central role ahead of Jesse Lingard with Mata and Henrikh Mkhitaryan on the flanks.

In theory, this is a way many United fans would like to see their team set up; with more speed, energy and penetration. Even when Ibrahimovic was scoring a remarkable 28 goals last season, his presence encouraged a more predictable approach that played to the Swede’s strengths rather than those of his young teammates.

Lukaku aiming a header at the Saints goal shortly before he suffered his worrying head injury 

Unfortunately for United, it is not so easy to switch seamlessly from Plan A to Plan B. Let’s not forget this was the first time they have played in the Premier League without Lukaku all season it showed. Too often they launched high balls forward as if the Belgian was still on the pitch.

Lingard, whose markedly improved form of late had brought six goals in seven games, was unlucky not to win a penalty when his breakaway and flick ended with Maya Yoshida knocking the ball down with his arm. The England international also headed inches wide from Mkhitaryan’s only decent cross of the first half.

Had the Armenian’s delivery been as good as Ashley Young or even Mata, United could have taken the lead before half-time against a spirited Saints side who had their fair share of chances at the other end.

But on the whole they struggled to create any clear opportunities, and it needed a fantastic save with his feet from David De Gea – not to mentioned a little bit of luck – when Shane Long was presented with the best chance of the second half.

The introduction of Anthony Martial, another young player with speed and skill, roused the Old Trafford crowd in what was also an awkward moment for Mkhitaryan, the man he replaced. The former Borussia Dortmund midfielder was really poor here on his first start for nearly two months, having clearly lost Mourinho’s trust. There was little said between the two men as Mkhitaryan walked off, and with the transfer window opening on Monday, it’s possible that Old Trafford may have seen the last of him.

Paul Pogba did have the ball in the net 10 minutes from time only to see it ruled out for offside, so Mourinho was left to contemplate a third Premier League draw in a week as Manchester City gallop away into the distance.

As the boos rang out at the final whistle and United head for Goodison facing the prospect of being without Lukaku and Ibrahimovic, they will have to do a lot better than this.