If this really was an audition for Europa League final places, then Sergio Romero has confirmed his name on the Manchester United team sheet. Granted only a second Premier League start of the season – in place of the rested David de Gea – the goalkeeper not only saved an early penalty from Manolo Gabbiadini, he kept out everything else Southampton threw at him.

There were several eye-catching saves, not least from James Ward-Prowse’s perfectly guided effort on the hour and, having started in each of United’s Europa League knockout ties, Romero has his eyes on the final against Ajax in Stockholm next Wednesday.

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Mourinho has said he will start with him and Romero’s CV has certainly come to read well. This was the Argentina international’s 17th appearance of the season in all competitions and his 11th clean sheet. He has been beaten only six times and, when he has featured, United have won 13 and drawn four.

It might sound strange for a manager to consider starting his No2 goalkeeper in a European final, particularly given De Gea’s status, and it will raise the inevitable questions about the Spaniard’s future at Old Trafford. Real Madrid continue to cast covetous glances but Mourinho explained his goalkeeping options were not normal and it was not difficult to see why he has such faith in Romero.

“I don’t go with the ‘if he [De Gea] leaves’ because I hope he doesn’t leave,” Mourinho said. “We need two top goalkeepers and a top young goalkeeper [De Gea] and this is what we have. We have the Spain and Argentina national team goalkeepers and, for me, it’s amazing, especially because of their personal relationship. I never saw two goalkeepers to be so friendly. Everything has gone very smoothly during the season. Everyone was accepting my decisions.”

It said everything about United’s performance and ambition that the goalkeeper was by far their best player. It is now certain the club will finish with their lowest number of Premier League wins in a season but Mourinho was more concerned with avoiding injuries before the Ajax final. Worryingly for him, Marouane Fellaini came off with a hamstring problem, which the midfielder assured him was not a tear, while Juan Mata also took a knock.

“Mata was just contact, no problem at all,” Mourinho said. “Marouane says that, from his experience, he doesn’t think it is a tear. So, trusting in him, he should be OK.”

Claude Puel had needed a victory given the uncertainly surrounding the Southampton manager’s job prospects and he deserved one on the balance of play. But his team lacked cutting edge and their goal drought at St Mary’s goes on. For the first time in the club’s top-flight history, they have failed to score in four straight home games.

Puel remains on thin ice and it has reached the point where it would be a surprise if he survived his end-of-season review. It had sounded ominous to hear Ralph Krueger, the chairman, refuse to offer him the slightest of assurances on Tuesday and, equally, it was not a good look when the home crowd jeered his late substitution of Ward-Prowse. There was a smattering of boos from them at full-time.

Puel’s likely dismissal would look harsh if he finished in eighth position and when it is factored in that he took Southampton to only the sixth major cup final of their history. Even Mourinho admitted they did not deserve to lose the EFL Cup to his team. But the stodginess of Puel’s football has worked against him, even if this was not a bad performance.

The penalty was awarded in the fifth minute for a handball against Eric Bailly, even if it looked to have been outside the area, but Romero sprang low to tip away Gabbiadini’s kick. It was Southampton’s third penalty miss in five matches. The first half also featured Bailly working Fraser Forster after a storming run into the box and Anthony Martial dragging wide of the far post.

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Southampton raised the tempo at the start of the second half and Romero distinguished himself further. Dusan Tadic’s free-kick exploded like a shell but Romero beat it away while, shortly afterwards, he did likewise with a low shot from Oriol Romeu, who had been teed up by Nathan Redmond’s cute cutback.

Southampton could lament a miscued volley by Jack Stephens at the far post, following a corner, while Romero made his best save to keep out Ward-Prowse’s curler from the edge of the area. His composure throughout was fortifying.

United looked wobbly, their defensive efforts last-ditch but they flickered towards the end. Chris Smalling led a break up the right and, when he crossed, Wayne Rooney held it up before laying off to Mata, who sent his shot high. Martial went the closest for the visitors with a trademark curler for the far corner. It came back off the inside of the post. Victory would have flattered United in the extreme.