Manchester City defender Eliaquim Mangala still finding his feet in the Premier League but time is of the essence

A frustrating week for Manuel Pellegrini was compounded by Manchester City's 2-1 defeat at Upton Park as Diafra Sakho secured three points for West Ham with his sixth strike in seven Premier League matches.

After watching his team throw away a two-goal lead in Moscow on Tuesday, Pellegrini would have expected a spirited response in East London to close the five-point gap to Chelsea. Instead, City were repeatedly overrun by an organised and committed Hammers side who now stand just a point behind the champions. City will be eight off the pace if Jose Mourinho can mastermind a victory at Old Trafford on Sunday.

Manchester United are City's next opponents on November 2 and, despite going into the game as favourites, Pellegrini will be wary of the attacking talents Louis van Gaal has at his disposal - particularly given his team's recent issues with defensive resolve. City have kept only two clean sheets in nine Premier League matches this season and none in three Champions League outings as they remain without a win in Group E.

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After beating Liverpool to the title in May, there was one obvious area in which City needed to strengthen in order to kick on in the current campaign: centre-half. Martin Demichelis, although a better buy than many have argued, is too old to form a long-term partnership with Vincent Kompany, while Matija Nastasic has rapidly faded out of the first-team picture under Pellegrini. Hindered by Financial Fair Play regulations, the club made a new centre-back the top priority.

Eliaquim Mangala quickly became Pellegrini's leading target, having impressed for Porto in the Champions League and, crucially, being able to operate on the left side of defence to slot in next to Kompany. Despite Mangala failing to command a starting spot for France at the World Cup, City were certain this was their man, and remained committed to concluding a deal amid protracted third-party negotiations and a fee that eventually climbed to £32m. The big question, however, is whether the most expensive defender in Premier League history has been worth the hassle.

Six games into his spell, Mangala has arguably been at fault for four goals - including West Ham's first on Saturday as he was shrugged off far too easily by Enner Valencia. The 23-year-old regained his composure in the second half, but the damage was done in an opening period pockmarked by alarming errors, both positionally and in challenging for possession.

Mangala needed a solid performance more than most after City's midweek slip, having been guilty of losing Ahmed Musa in the build up to CSKA Moscow pulling a goal back. Similar to a recurring error in Kompany's play, the 23-year-old stepped up to try and win the ball, allowing Musa to slip in behind and collect the pass before crossing for Seydou Doumbia to convert. It proved to be a costly mistake.

It is perhaps curious that Mangala's best display came in an impressive debut against Chelsea at the Etihad, but since then progress has been slow. A nightmare trip to Hull saw the Frenchman score an own goal and concede a penalty, allowing the hosts to briefly recover before City eventually restored their lead. "These things can happen," said Pellegrini. "He was very unlucky with the own goal and after that he arrived late for the penalty, but we continue to trust him because he is a very good player who had very bad luck."

Mangala has admitted that he is taking time to adjust to the physicality of English football, telling L'Equipe recently: "There is a difference. In duels (in Portugal), the attacker will often pull out of the challenge. In England, it is completely different. You have to be prepared, because the attacker will not pull his foot out. If he can leave you on the floor, then he will leave you on the floor. And you, if you pull out of the challenge, you can't be surprised if you get a bad reaction from the crowd. If one day you're playing at 80%, you're not going to manage."

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It is encouraging for City that Mangala understands the demands of the Premier League as he embraces the steep learning curve. His comments are reminiscent of Nemanja Vidic reflecting on his difficult start at United when his career in England ended in May. "For us (Vidic and Patrice Evra), the league was much quicker, the players were much stronger and in the first few weeks I found that really hard," Vidic told The Telegraph. "I remember Patrice saying 'can we succeed here? Maybe it is better for us to go back to the places we were at before.' But afterwards, we start training harder and we got used to it."

Pellegrini will hope for the same improvement from Mangala, although his patience will have been tested by a difficult week. It would not be a surprise to see Demichelis return to the starting line-up in the Manchester derby given the enormous pressure of the occasion. Perhaps it is time for Pellegrini to place his trust in experience rather than potential as he seeks to rein in Chelsea's commanding lead.