Five things we learned as Manchester United laboured to win against Rostov

Jose Mourinho's men progressed to the last-eight after a drab encounter at Old Trafford

Jose Mourinho's back three looks set to stay for the time being (Getty)

Back-three system may be here to stay, but it needs tweaking

For the third successive game, Jose Mourinho lined up with a back three, despite less-than-ideal returns in the first-leg and against Chelsea on Monday night. It is quite a departure for the Portuguese, whose various systems throughout his career have always rested on a rigid four-man line in defence.

This new-look formation offers the advantage of pushing their creative wing-backs further up field but if it is to stay, Chris Smalling should not be its ‘sweeper’, the central defensive point. The England defender’s form has massively dropped over the past year or so, and he looked lost at times here, uncertain of what this new role demanded of him.

Ibrahimovic remains integral to this team

United’s support is more ambivalent about Zlatan Ibrahimovic than you might think. It cannot be denied that the Swede has talked the talk and walked the walk this season, but his tendency to miss a surprising number of chances grinds many a gear at Old Trafford.

Yet on his return to the side last night, Ibrahimovic showed why he is currently integral to Mourinho’s current line-up. His interplay with the other forwards around him is often where United’s best opportunities come from, and so it proved here, his deft flick teeing up Juan Mata for the deadlock-breaking goal.

He is simply his side’s most dangerous player. They need him to sign that contract extension.

Valencia is Mourinho’s most consistent player

As the award season approaches, many will probably plump for Ibrahimovic when asked to pick the Sir Matt Busby Player of the Year. To do so, however, would be to overlook Mourinho’s most reliably impressive player.

It has been a long road back for Antonio Valencia since he gave up the No 7 shirt he was awarded after his brilliant 2011/12 campaign for United. It looked over for him after that, as though he lacked the mentality to recover his career at a club of such stature.

Yet this season, even on drab nights such as this, the makeshift right-back has consistently created chances at one end and cut them out at the other. He deserves a second award and this time, make sure he keeps the shirt number.

If Pogba is set for spell on the sidelines, that may be a good thing

Paul Pogba is desperate to impress - the hairstyle says that much - and that is why it was disappointing to see him depart the pitch injured after 48 minutes.

It simply is not clicking for the midfielder at the moment, and his status as the world’s most expensive footballer is only focusing more attention on his middling performances.

Pogba has not looked like a once-in-a-generation talent this, nor has he looked like a ‘fraud’, as so many of his critics would have it. More than anything, he looks tired, so perhaps this injury will be something of a blessing. A spell out of the spotlight is overdue.

Romero can be counted on, if needed

If there were any positives to take from those brief moments where Rostov threatened to spoil United’s evening, it was that Sergio Romero stood firm and denied the Russian Premier League side.

Mourinho’s back-up goalkeeper twice pulled off acrobatic stops to spare the blushes of his team-mates, first from Sardar Azmoun and then from Aleksandr Bukharov.

The performance that Romero is remembered for, a nightmarish display at Swansea in August 2015 when it looked as if he would be United’s No 1 for the foreseeable future, is now well and truly behind him.

As whispers over David de Gea’s future resurface, Romero showed he can be counted upon if needed.