Manchester City made a real statement of intent on Sunday by beating champions Chelsea FC 3-0 at the Etihad stadium, with goals from Sergio AgA?ero, captain Vincent Kompany as well as Brazil international Fernandinho.

It took less than half a minute for Manchester City to expose the Chelsea FC back-line, when a delightful through ball from Spaniard David Silva saw Argentine superstar Sergio AgA?ero one-one-one with the Blues’ stand-in keeper Asmir Begovi .

Although Jos Mourinho’s men were let off the hook on that occasion, it did highlight what is arguably the single most fundamental problem with the current Chelsea FC team – the defence. It might sound counter-intuitive given that defensive solidity is the bedrock of Mourinho’s philosophy, but the Citizens’ ruthless exploitation of an increasingly desperate Blues’ defence – particularly in the first half – indicates that the former Real Madrid manager has some tough decisions to make going forward.

One such decision was, in fact, made at half-time in Sunday’s game, when Mourinho replaced captain John Terry with Kurt Zouma – the first ever time the Englishman has been substituted when playing under the Portuguese. The explanation provided was one that made a degree of sense – Mourinho sought to use Zouma’s pace to try and thwart a City side looking to play on the break in the second period.

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A lack of pace has long been a problem with the Chelsea FC defence, but one that has been largely covered – partly due to sound defensive positioning, but also due to the Blues’ pressing in midfield which has denied opposition the time and space to thread balls through to strikers playing off shoulder of the last defender.

The number of times the Chelsea FC defence has been cut open in the first two games, however, suggests that Mourinho might dive head-first into the market for a certain John Stones – a player whose youth, athleticism and ability on the ball is exactly what the Blues need.

Mourinho admits defending in the first half was not up to the mark

Jos Mourinho has suffered heavy defeats in his career, but few would have stung as much as Sunday’s 3-0 humiliation at the hands of his rival Manuel Pellegrini – a result made worse by the negativity surrounding the club following the opening-day draw against Swansea City and a media storm in the aftermath of a row with the medical staff.

Speaking after the game, he is quoted by the Chelsea FC official website as saying: “In the first half the best team was winning, in the second the best team was Chelsea. We didna t score and if the 1-0 result after the second half was an unfair result in my opinion, imagine 2-0 or 3-0. They were the best team in the first half, they controlled the game and created chances. We were comfortable on the ball but defensively we were quite fragile.

“Ia m the first one to be disappointed when all week you are speaking about movements, directions of the game, mechanisms, Agueroa s runs and positions, wingers coming from the outside to the inside, and then after 10 seconds he [Aguero] is in the face of Begovic. We were defensively poor in the first half.”

John Terry substitution for tactical purposes

Despite having a fantastic 2014-2015 season where he was undoubtedly the best centre-back in the league, Chelsea FC captain John Terry has not started this season on a positive note. The 34-year-old has looked every bit his age, his lack of pace being compounded by what appear to be lapses in concentration.

His substitution at half-time caused a stir simply because of what it could potentially mean for the rest of the season. Terry is likely to return to the starting line-up next weekend, but it is difficult to imagine the veteran being able to consistently maintain the levels he did last season – if and when he does actually get back to that level. Having conceded five goals in two games – with Branislav Ivanovi being the worst in a shaky back-four, it would be extremely surprising if Mourinho did not return with a sizable offer for John Stones.

Mourinho said of the Terry substitution (via Chelsea FC website): “I had to decide on John or Cahill because Zouma is the fastest player we have in our defensive line. When you are going to play with a high block and you are asking your players to do that, the fastest player has to be on the pitch.

“The point was not to bring John out but to bring Zouma in. I wanted to have my fastest player on the pitch and not on the bench because I knew they would play counter attack and long balls into Aguero. I knew I wanted my team to be dominant and to play in the opposition half.

“Ia m the one that plays John every game, made him captain, brought him back from a difficult situation with other managers. Ia m the one who has the right to look at the game and say I want Zouma on the pitch.”

Chelsea FC have shown great foresight in identifying and securing players for the long term in the midfield and attacking areas. It might be a little overdue, but Mourinho must ensure he does the same for the defence before it is too late.