The 25-year-old has not been convincing at the back for the Blues and Lampard has dropped him at times this season, replacing him with the even less reliable Willy Caballero. Chelsea have been linked with a string of top-class goalkeepers, with the most blockbuster of moves available to them being Oblak.
The Slovenian remains under contract until 2023, though, and Atletico have no desire to let him go, so it would take a huge offer from Chelsea and for the player to push for a switch to the Premier League.
The 23-year-old Manchester United keeper has caught the eye on loan with the Blades, but the Red Devils would demand an enormous price for someone they see as their long-term replacement for David de Gea, especially when it is a rival asking about the price.