Chelsea had already given their approval for Fernández to join City on deadline day, judging it would be unethical to withdraw from the deal at the last moment, and made a separate attempt to bring in Monaco's Lamine Camara as cover in the same window.
Monaco's refusal to sanction that move, taken at a very late stage, left Xabi Alonso's midfield short of options heading into the new season with no replacement in place for the departed Argentine.
According to Sport, Chelsea will now attempt to resolve the situation in the January window, with Barrios identified as their preferred target. The 23-year-old is viewed at Stamford Bridge as the ideal long-term partner for Moisés Caicedo in central midfield, combining physicality with creativity in a way Alonso believes suits his system.
Sources in England have suggested the west London club are prepared to offer around €100m to prise him away from the Metropolitano.
Persuading Atletico Madrid to sell in the middle of the season looks a significant obstacle. Barrios is regarded as a cornerstone of Diego Simeone's midfield and carries a release clause reported at €500m, a figure that underlines the size of the challenge facing Chelsea if they want to complete a deal without Atlético's full cooperation.
Last season marked something of a breakthrough campaign for the Spain international, though a hamstring injury cost him the closing weeks of the campaign and, subsequently, a place in Luis de la Fuente's World Cup squad — a setback that came at a cost, given he had looked a near-certainty for selection before picking up the injury.
Negotiations with Atlético in January are expected to be difficult. The Madrid club demonstrated over the summer, particularly during the extended saga surrounding Julián Álvarez, that finances are no longer the limiting factor they might once have been under the ownership of Apollo, and Chelsea are likely to find them a tough negotiating partner.
For now, though, the west London club appear to have settled on their man: Pablo Barrios is Chelsea's choice to replace Enzo Fernández, output completing the deal will not come cheaply.