In the end, perhaps it was just inevitable that Zlatan Ibrahimovic would insist on having the final say. Southampton had given everything to drag themselves level and, for long spells, it had felt as if Manchester United could fall to one of the more improbable comebacks of any Wembley final. It would need a flint heart not to sympathise with the losers but this is why José Mourinho was so determined to sign Ibrahimovic: the decisive moments on the occasions that really matter.

The clock had ticked into its 87th minute when United’s new talisman applied the header that spared them the possibility of losing a game they had led 2-0. The same player had opened the scoring with another reminder of his eye for the spectacular and his late winner brought a measure of relief as well as jubilation, bearing in mind the determination of Southampton to fight back and give their opponents an almighty scare.

Amid all the fireworks and ticker-tape, Mourinho is certainly entitled to wonder why his side made life so difficult for themselves once Jesse Lingard had doubled their lead seven minutes before half‑time. Southampton deserve immense credit because the paradox of this match is that United, as Mourinho acknowledged, were the inferior team. It is possible to lose while also playing with distinction and Southampton did just that on a day when Manolo Gabbiadini, their £14m signing from Napoli, had his own credentials to be recognised as the game’s outstanding performer.

Gabbiadini’s first goal arrived in first‑half stoppage time, followed by another three minutes into the second half, and he could conceivably have been reflecting on a hat-trick and an entirely different outcome had it not been for a trigger-happy linesman raising his flag to rule out an early effort, with the game goalless. United supporters of a certain generation have spent many years grizzling about Bobby Stokes’s winner in the 1976 FA Cup final – a goal once described as so far offside Stokes was almost in Hendon – but this one went in their favour and it will be little consolation for Southampton that football’s law‑makers convene at Wembley on Friday to discuss the merits of video replays.

Nobody will ever know what would have happened had that goal stood but Southampton left with a smouldering sense of injustice. Eight minutes later, Oriol Romeu scythed down Ander Herrera 30 yards from goal and Ibrahimovic’s spearing free-kick deceived Fraser Forster to give United the lead in a game that, by rights, they should have been losing.

Ultimately, though, there was another reason why Southampton’s players finished the match with only despondency. They moved the ball around confidently, there was not a hint of stage-fright in their first final since 1979, and Nathan Redmond and James Ward-Prowse were difficult, elusive opponents on either side of Gabbiadini. Defensively, however, they were unable to demonstrate why they had become only the second team in history – Tottenham in 1981-82 being the other – to reach this final without conceding a single goal.

Forster chose a bad day to suffer a lapse for the opening goal. Ibrahimovic was afforded far too much space for the winner and Lingard’s goal was another case in point. Southampton had plenty of players in defence without anyone getting close to the ball as Juan Mata, Anthony Martial and Marcos Rojo exchanged passes to create the opportunity. Lingard’s precise finish went through the legs of Maya Yoshida and, at that stage, it was difficult to see any other conclusion than a straightforward United win.

Instead, Claude Puel’s players shook their heads clear and came back with a level of determination and skill that seemed to surprise their opponents. Gabbiadini’s first goal came from close range, clipping the ball through David de Gea’s legs when the United goalkeeper might have been better advised going for Ward-Prowse’s cross with his hands rather than his feet. Puel had been facing the ordeal of a half-time team-talk at 2-0 down, but the goal dramatically changed the complexion of the match and when Southampton came out for the second half they were quickly on the attack.

As well as Gabbiadini’s second goal, Southampton could also reflect on the header Romeu, getting the better of Paul Pogba, directed against the post from one of Ward-Prowse’s corners with the score at 2-2. Gabbiadini’s movement and anticipation made him a regular threat and his equaliser was superb, waiting for a header to drop in a congested penalty area before firing a swivelling volley past De Gea, despite the close proximity of two United defenders. Mourinho described it as “phenomenal”. It was Gabbiadini’s fifth goal in three games for his new club and from that point onwards there was no doubt Southampton fancied themselves to complete the recovery.

United looked disorientated and, just before the hour, Martial and Pogba could be seen leaving a cross to one another when both players had the chance to score. Mourinho has never lost a domestic final but that record was under serious threat and it was not until the final 20 minutes that his team started to threaten again.

Even then, there were times when Chris Smalling and Eric Bailly looked vulnerable in the centre of United’s defence. Pogba had a difficult afternoon and it was tempting to see if Mourinho was wearing Puel’s initials on his top when he started his press conference afterwards by expressing his sympathy for Southampton’s manager. They were, he said, the better side and deserved extra time.

What Southampton did not have, however, was someone marking the player who might yet win the season’s individual awards. Ibrahimovic was alone, six yards out, when Herrera’s cross picked him out, heading in his 26th goal of the season to put Mourinho level with Sir Alex Ferguson and Brian Clough with his fourth League Cup triumph.