Atletico delegate general Miguel Angel Gil Marin claims Chelsea made a summer offer of around £41million for the Argentina international, who played in the defeat to Carlo Ancelotti's men last week at Stamford Bridge in the Champions League.
Gil Marin told Marca: "If they want to make a better offer, Atletico will assess it."
Meanwhile, midfielder Joe Cole is happy with how contract talks are developing about staying at Chelsea beyond the end of the season when his current deal runs out.
Cole, who stepped up his rehabilitation from his knee injury with an appearance against Blackburn on Saturday, told the Evening Standard: "We're talking at the moment. I love it here. The club want me to stay, so there should be no problems and I feel like this is my home.
"There is so much to look forward to and I am so excited about just playing football for Chelsea and England.
"There are a lot of ups and downs in football and I've had eight-nine months of sitting on my backside watching so the game against Blackburn was like a breath of fresh air for me."