The beauty of a sliding doors moment is that you never quite know what an alternative reality had in store for you. Things might have been different, they might even have better, but it’s all guesswork.

That doesn’t stop us from thinking about the what-ifs. From wasting time wondering about a vision we will never get to see. Erik ten Hag couldn’t help himself last season, thinking aloud in May about how different Manchester United’s season might have been had a decision been different back in September.

These little moments can become an obsession. For Ten Hag, it was United’s late defeat at Arsenal in their fourth Premier League game of the season. What started simply as a moment of frustration morphed into something more substantial as the season came off the rails.

That’s why he was still talking about it seven months later. In Ten Hag’s version, United were denied an 87th-minute penalty at 1-1, then scored through Alejandro Garnacho only to see the goal disallowed for the tightest of VAR offside calls. Arsenal went up the other end and won the game themselves, with a goal United felt should also have been ruled out.

It was the turning point that never was. One of the more encouraging performances of the season ended in defeat. Had it ended in victory maybe things would have been different. Not just the three extra points, but the belief it would have instilled in the squad.

We’ll never know now, but if that was Ten Hag’s example of a turning point that cost United, in the fourth game of this season he got one that will undoubtedly benefit them.

“Confidence is a great thing,” Ten Hag said back in May. “This season we almost didn’t get the turning point, now we can make a step through.

“I remember the Arsenal game and in the 87th minute, I thought we won it. That could have given a big lift to the team. We never got this.”

They’ve got it now. For half an hour at St Mary’s United had been dismal. They had been outplayed and outfought by Southampton, who looked the more threatening side. They should have made their dominance pay and when Diogo Dalot fouled Tyler Dibling, they had the perfect chance to cash it in.

Instead, Andre Onana enhanced his reputation as a penalty-saving expert, going the right way to keep out Cameron Archer’s tame effort. United suddenly pulled themselves together. Joshua Zirkzee’s fizzing effort was tipped round the post by Aaron Ramsdale and from the resulting corner Matthijs de Ligt headed them in front.

Soon after the lead was doubled and the game was done. Marcus Rashford’s fine finish put the game out of sight and eight minutes after they faced that penalty, United now had a level of control they would never relinquish, with Alejandro Garnacho icing the cake deep into stoppage time.

Had Archer been more convincing with his penalty, or had Onana gone the wrong way, it’s not difficult to imagine what might have happened. A Southampton side that had already grown in confidence would have seen their belief fuelled further. United didn’t look a side capable of a comeback. Thankfully, they never had to contemplate one.

If United had lost this game then Ten Hag’s position would have been in peril. As Archer lined his penalty up, that looked a distinct possibility. A couple of hours later the Dutchman’s smile lit up St Mary’s.

This has to be the turning point he felt he was denied last year. After two defeats in their first three Premier League games, United were in an uncomfortable position. This buys them breathing space.

More than that, the way they reacted to Onana’s penalty save, the way they sensed their moment to go up a level, should be when it clicks for them.

On the south coast, they never looked back. Now they need to do the same for the rest of the season.