'He was never City's first choice' - analysis
Rob Schofield, BBC World Service
Dias is the latest City recruit to have started life in Benfica's Seixal academy, but unlike compatriots Bernardo Silva and Joao Cancelo he won't be afforded a season to bed in.
Since Kompany's departure in 2019, City have needed an authoritative, combative, right-footed centre-back and the defeat by Leicester showed they cannot wait any longer.
Dias was not City's first choice. They wanted the finished article but, with their relationship with Napoli irreparable after the Italian club sold Jorginho to Chelsea in 2018, a deal for Koulibaly for less than £70m always looked unlikely.
This may prove better business though. Dias is six years younger, cheaper and offers City the opportunity to offload Otamendi, 32, at the same time.
Koulibaly's form has fluctuated a great deal depending on his defensive partner, while Dias has proven himself as a consistent performer in the three years since breaking into the Benfica first team, as well as a leader - which City sorely need.