It's difficult to say where it all went wrong for Joe Hart at Manchester City. He was once the rock on which two title-winning teams were built, capable of pulling off incredible saves to keep his side in the lead in tight games and he played a huge part in the club's successes in 2012 and 2014.

He's now staring down the barrel of the threat of his career frittering away much earlier than it should have done.

On Sunday, Pep Guardiola takes the newly crowned champions to West Ham's Olympic Stadium. Hart, the goalkeeper he ditched almost instantaneously after arriving in Manchester, will play no part, still on loan from the Etihad and unable to feature against his parent club -- but he must now be wondering what might have been if things had gone differently back in 2016.

When he returns to City at the end of the season, he'll have a year left on his contract and, given his performances for the Hammers this season, he may have few clubs willing to take a chance on him. He won't be usurping Ederson and he won't be happy to be a back-up option -- but which clubs would be willing to sign him based off his erratic performances in the last 18 months?

It's a stark turnaround for the 31-year old, who could reasonably have been expecting to be England's No. 1 at the World Cup just a couple of years ago. He's a shadow of the man who helped Roberto Mancini and Manuel Pellegrini to domestic success when they were in charge of City.

With the horror-show performances he's put in at times this season, it's easy to forget just what an integral part of City's team he was. The idea that any goalkeeper could sit behind one of the best defences in the top flight and look good -- an accusation often made at Hart -- was thoroughly torpedoed last season, when Claudio Bravo looked like he was being played out of position.

When Mancini began his first full season in the dugout, he opted for the younger Hart over the experienced Shay Given. He'd excelled in a year away at Birmingham, winning a place in the PFA Team of the Year for 2010, and the manager's faith was instantly repaid. The goalkeeper pulled off a string of brilliant stops to deny Tottenham what should have been a comfortable win in a 0-0 draw on the opening day of 2010-11.

That season, as City were closing the gap on the teams above them, Mancini specialised in clean sheets. A defensive partnership of Vincent Kompany and Joleon Lescott began to flourish and, with Hart behind them, City were always a good bet for a shut-out. It helped them win the FA Cup, with tight 1-0 wins at Wembley over Manchester United and Stoke in the semifinal and final respectively -- and Hart pulled off big saves at big moments in those games.

He did the same for Mancini's side as they went on to lift the title the following season, although it's true City did become less reliant on the defence as they began to score far more goals to win games themselves.

Joe Hart has lost his place at Manchester City and isn't assured of his place in the England squad for the World Cup. Clive Rose/Getty Images

Those were undoubtedly Hart's best years at City. He was brimming with confidence, able to command his box with authority and he was decisive when it came to coming for the ball or leaving his line. He won the Golden Glove award three years running.

Even after a dip in form under Pellegrini, he came back stronger. The manager took him out of the firing line for a short while, reinstating him a few weeks later -- and it inspired a string of good displays that culminated in a title-winning save from Steven Naismith in a high-pressure 3-2 win at Everton.

In his prime and on his day, Hart was phenomenal for City. A 1-1 draw with Borussia Dortmund or in a 1-0 defeat to Barcelona are two of his stand-outs -- he often saved his best for the European stage, when the team as a whole often flattered to deceive.

It makes the version of Hart that City fans watch from afar now all the more sadder. Guardiola's choice to move on the goalkeeper wasn't popular last season, but it's proven to be the correct call -- the error last term was in the choice of replacement, not the decision itself.

The weakest area of Hart's game was always his kicking. It seemed to deteriorate over the course of the six years he was City's first choice, to the point where his only appearance under Guardiola in 2016-17's preseason saw him skew the ball left, right and centre with damning regularity in a 3-2 defeat to Arsenal in Gothenburg. He also seemed to refuse point blank to play the ball short, which was probably the final nail in his coffin and saw Willy Caballero picked ahead of him for the Premier League opener.

It's a sad sight for City fans to watch Hart struggle as he has been. The supporters, who always had a strong connection with the goalkeeper, would prefer instead to remember his fine displays of years gone by.