John Stones’s thigh injury makes Manchester City’s failure to sign a centre-back in the summer all the more conspicuous. Even before the 25-year-old was ruled out for up to six weeks with a problem suffered in training on Tuesday, Pep Guardiola had difficulty concealing frustration when asked about strengthening the position during the winter window.

The question was posed on Friday because the manager had confirmed his defensive linchpin, Aymeric Laporte, would be unavailable until the spring because of a damaged knee. After attempting to dismiss the signing of a stop-gap for Laporte in January as media speculation, Guardiola revealed his true feelings. “But we could not invest in this situation in the summer, we cannot do it in winter,” he said. “That’s why we are going to wait with the players we have, with the two central defenders,” meaning Stones and Nicolás Otamendi.

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Except now Guardiola has only Otamendi as a frontline centre-back and he can be shaky, as the mistake that gifted Norwich what proved their winner on Saturday showed.

Factor in how Vincent Kompany’s departure in May had caused Guardiola to target a replacement and the policy not to recruit one appears suspect and has backfired. The manager’s primary target was Harry Maguire but City’s refusal to pay Leicester’s £80m asking price did not shift. The champions valued Maguire at no higher than £65m and so lost out to Manchester United, who did meet the fee.

This is reminiscent of how City would not pay Southampton the desired £75m for Virgil van Dijk, which allowed him to move to Liverpool in January 2018. Given how the Dutchman has been key in the emergence of Jürgen Klopp’s team as title contenders and in last season’s Champions League triumph, City and Guardiola must surely regret not meeting Southampton’s demand.

Stones’s injury will surely cause the Catalan to press the hierarchy to recruit a centre-back in January. When he wanted one in the summer Guardiola – strangely – ended up with a full-back, João Cancelo. The Portuguese has played one minute for City, suggesting his position was not a priority.

A key managerial mantra runs: “Strengthen from a position of strength.” For some reason, the domestic treble winners failed to adhere to this at centre-back. It could prove costly as they seek to defend the Premier League title and claim a first Champions League crown.