Liverpool's win over Manchester City provoked one question: what if Sadio Mane had not been sent off?

Jurgen Klopp saw it as a Sliding Doors moment and on Sunday, we were given a glimpse of what might have happened had the Senegalese winger stayed on in September

As Sadio Mané’s strike ripped past Ederson into the top-left hand corner of the Anfield Road net, it answered the outstanding question of this Premier League so far.

Can Manchester City be beaten? Yes, as it turns out, they can. Emphatically so, too.

Mané’s goal, Liverpool’s third on Sunday, may not have ultimately made the difference in their 4-3 victory over the otherwise-invincible Premier League leaders but at the time it was scored, it felt significant.

Once it had crossed the line, a two-goal lead had been established – clear daylight – and for the first time this season, Pep Guardiola’s seemingly-unstoppable side required three to win.

It was also significant for the identity of the scorer, though. It was Mané, after all, who was at the centre of the pivotal moment the last time the two sides met, a moment that – even after this first league defeat of the season for City – is still likely to be the only decisive one in this year’s title race.

On Friday, at his pre-match press conference, Jürgen Klopp harked back to it. “We were the slightly better side, created slightly better chances, but an open game,” he said of the reverse fixture at the Etihad last September. “Both teams not defending fantastically but both on the front foot.”

Then came referee Jonathan Moss’ correct decision to show Mané a red card for unintentionally but recklessly kicking Ederson in the head while contesting a high, bouncing ball.

What followed was total capitulation on Liverpool’s part and a 5-0 defeat. Klopp admitted it was one of the “stranger” games of his career. The red card had “obviously changed the game”, he said, “and maybe for Manchester City, the season.”

That last comment from Klopp was particularly interesting. It suggested that in the months since that match, he has asked himself: ‘What if?’

What if, for example, Mané had controlled the ball first time without letting it bounce towards Ederson? What if he had then rounded the onrushing goalkeeper and stroked the ball into an empty net? What if, with the scoreline level, two evenly-matched teams in a tight contest had shared the points that day?

Would City have still gone on to win their next 16 league games, racking up heavier and heavier victories, building a significant lead at the top of the table? Maybe. Would Liverpool have suffered their worst period of form of this season so far in the weeks immediately after? Maybe. Or maybe not.

Klopp’s words on Friday indicated that he feels that Mané’s dismissal was the significant moment of the season, a Sliding Doors moment if you like. Though the decision itself was a fair one, the Liverpool manager believed the scoreline it produced was false, and one that carried consequences for the campaigns of both clubs.

He went on to ask his players to “clarify something” against City on Sunday, and though he did not specify what exactly, the suggestion was that they should do everything possible at Anfield to make September’s 5-0 drubbing seem like an aberration.

They did that and then some. City will still win the title and if both teams are at their very best, Liverpool remain some way below Guardiola’s side, but they are not inferior to the tune of five unanswered goals. On Sunday, they proved that and also showed they can not only compete with but beat a team that may yet prove to be the best the Premier League has ever seen.

More than anything though, the game was a reminder that seasons, never mind matches, can swing on single moments. And while it was a win that answered the biggest question of this Premier League season so far, for Klopp, it will also have provoked many more questions about what could have been in that fixture at the Etihad.

What if an 11-man Liverpool had produced the same performance there in September? What if indeed.