Robert Sánchez's performance in Monday's 3-2 win over Fulham prompted the rethink. The Spaniard let a tame effort from Josh King slip through his grasp at the near post, then spilled another shot into a dangerous area in the second half, allowing Gonzalo García to score.
Sánchez has been Chelsea's first-choice goalkeeper since his £25m move from Brighton in 2023, and the club's hierarchy had publicly backed him as recently as this week — head coach Xabi Alonso was asked directly about his goalkeeping options and voiced confidence in the group — making the swift move for Martínez all the more striking.
The 33-year-old Argentina international will undergo a medical in London before the move is finalised. He becomes available after Aston Villa signed Japan goalkeeper Zion Suzuki from Parma for close to £30m earlier this month, a deal that left Martínez as Unai Emery's clear second choice and prompted Villa to sanction his departure.
Martínez arrived at Villa Park from Arsenal for £20m in 2020 and went on to become one of the Premier League's most decorated goalkeepers, winning the 2022 World Cup with Argentina and lifting the Europa League with Villa only last season.
Chelsea also have 21-year-old Belgian goalkeeper Mike Penders among their options, a £17m signing from Genk who is in line to make his debut in the Carabao Cup against Luton Town on Thursday night after an impressive loan spell at Strasbourg.
Chelsea's co-sporting directors had previously suggested three senior goalkeepers would be excessive for a squad not competing in Europe this season, making Martínez's arrival a significant shift in that thinking as much as a response to Sánchez's form.
The move continues a busy pattern of dealings between the two clubs this summer, adding a further chapter to a relationship that has already produced several transfers between Chelsea and Villa this window.