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When City spent the best part of £130million on three new full-backs in the summer of 2016 it was quickly pinpointed as a key moment in the transformation of Pep Guardiola's side.

Full-backs are the fulcrum of many modern teams, especially those built by Guardiola, and Bacary Sagna, Gael Clichy and Aleksandar Kolarov simply weren't able to provide what Guardiola wanted in 2015/16.

So out they went and in came Kyle Walker, Benjamin Mendy and Danilo. It was their energy as much as their quality that transformed City. They were faster, younger and fitter than the players they replaced. They gave City a new thrust from deep, capable of overlaps or of adding to the numbers in midfield to dominate the ball and territory.

While Mendy's first season was cut short Fabian Delph performed admirably as his replacement and Walker's pace was a valuable asset down the right. City improved by 22 points from Guardiola's first season and while there were plenty of areas where they were better, it was the rejuvenation of the full-backs department that stood out.

As 2018/19 dawned it looked like it would be a similar story. Mendy was back from knee surgery and he was at his brilliant best in the early weeks of the season.

Guardiola was placing a greater emphasis on him and Walker making overlapping runs and Mendy's five assists in his first six Premier League games looked to be vindication of that approach.

The Frenchman set up both goals at Arsenal on the opening weekend of the season and while Guardiola admitted he was sometimes frustrated by him, his admiration for the role performed by both full-backs was clear.

"Mendy is Mendy -- he is what he is," he said. "Sometimes you want to kill him and sometimes you think, 'what a player!'

"With Kyle and him, we have that extra energy."

But midway through the season City's great strength is suddenly beginning to look like a weakness, posing Guardiola more problems than his frustration with Mendy's unpredictability.

On the left Mendy had already turned in a couple of questionable displays before undergoing knee surgery in November that sidelined him for another three months. While Delph and Aleks Zinchenko proved able deputies last term, they've been exposed this season.

(Image: Getty Images)

City's general domination last season might have restricted the ability of opponents to discover a weakness, but a year on Premier League teams are focusing their attacking intentions down City's left.

They've been no more reliable on the right, however. Walker's slump in form hit a nadir against Crystal Palace before Christmas and any suggestions his absence on Boxing Day was simply down to rotation were dispelled when he missed a second successive game against Southampton.

Walker played as a right-sided centre back in the World Cup but back at right-back for the Blues he's looked defensively uncertain this season. That has resulted in Danilo taking his place, but the jury remains out on the former Real Madrid man too. His contributions going forward remain impressive, but he's looked no more assured than Walker against the Foxes and the Saints.

Guardiola's full-back dilemma will continue to dominate his thoughts ahead of Thursday's crucial clash with Liverpool at the Etihad. With Delph still suspended after his Boxing Day red card and Zinchenko struggling defensively at Southampton his options on the left are particularly slim. Aymeric Laporte could move across, or Danilo could switch to the left and Walker could return on the right.

There's no easy solution for Guardiola though, and that shows how an area that City looked so dominant in has quickly become one that is a source of strength for the opposition.