Aguero in the spotlight as West Ham stun Manchester City

What a strange sport football can be. Manchester City were untouchable seven days ago. They'd just seen off their latest challenge thanks to an injury-time goal from a teenage striker, extending their immaculate start to the season. Five games in, five victories, 11 goals scored and none conceded, Manuel Pellegrini's side were top of the Premier League and top of the world.

Even some off-colour performances from Sergio Aguero had been papered over by superb contributions from pretty much everybody else on the pitch; even the 19-year-old Kelechi Iheanacho in the Argentine's injury enforced absence.

Now, the club are reflecting on back-to-back defeats and Aguero's form is under the microscope. Their fine run was derailed by Juventus in the Champions League on Tuesday but that could, in part, be put down to the Italians' European nous and some poor finishing on City's part, with the Argentina international on the sidelines.

Manuel Pellegrini's men had probably deserved to win that game but could not rely on their expensive forward line - namely Wilfried Bony and Raheem Sterling - to find the back of the net, while Gianluigi Buffon continued his claim to be remembered as the finest goalkeeper in history.

At least, it was reasoned, they could get back on track in the Premier League, where they had already shown themselves to be far better than the rest.

But it has happened again. West Ham have suddenly become the best counter-attacking team in the Premier League and, having won at Arsenal and Liverpool, repeated the trick here. But even after going 2-0 down in little over half an hour, City had plenty of chances to win it and it remains a mystery how they didn't emerge from this with at least their unbeaten run still in tact.

Before the City inquest begins it must be said the Hammers were terrific. Dimitri Payet is proving to be one of the buys of the season and Victor Moses's ability on the break is a real weapon. Diafra Sakho, who appeared to be a flash in the pan after his bright start to last season, looks rejuvenated. After half-time it was their defenders who earned the plaudits, bunkered into their own box and scrambling away anything that moved.

But that task was made easier by Aguero's quiet showing. If his touch in the build-up during the first half - linking up with his midfielders and defenders - was spot on, it deserted him in important positions.

Aguero skipped past the hot-headed Adrian at 1-0 down but clipped his shot wide of the post from just outside the box. On one or two occasions before the break he could not get the ball under his control when in sight of goal.

By the early stages of the second half desperation had set in. A couple of times he threw himself to the ground under minimal contact, and by the final 30 minutes it was Yaya Toure who took it upon himself to drive forward and get a leveller - he must have seen four or five shots blocked - while Bony came on for Sterling, who also struggled to make his mark, and became the focal point of his side's aerial bombardment.

The Hammers barely got out of their own box, and at times it reminded you of City's final home game of the 2011-12 season against Queens Park Rangers.

The difference then was that Aguero had his eye in. Since he helped tear Chelsea to pieces back in August that has not been the case. At Everton he faded after going close on a couple of occasions and at Watford he was off kilter, too. Damien Delaney's intervention meant his attempts to make amends at Selhurst Park were limited to 40 minutes last weekend and here he was worryingly off form again.

He is not in terrible form by any stretch of the imagination - Sterling made less of an impact on Saturday, which is to be expected of a youngster - but as the leader of this expensively assembled team much more is expected of him. The devastating finishing ability he has shown since arriving on these shores in 2011 has bred high expectations, and after Bony's wasteful display against Juve it was hoped the Argentine would return to the team and re-discover his eye for goal.

But he was out of sorts once again and, unfortunately for him, his team-mates could not find the answers as they had done before.

Pellegrini was in a foul mood post-match and he will know Aguero needs to get back to his best if this season is to deliver upon its early promise. There's every confidence that will happen, but for now this brilliant striker is struggling.