Gallagher is not due to fly out to the United States to join the rest of the Blues squad for the remainder of their pre-season tour but is instead set to return to Cobham.
The 24-year-old boyhood fan of the club will do so with his future up the air, having been afforded an extended break by Chelsea after England's run to the Euro 2024 final.
Gallagher is set to enter into the final year of his contract, making this summer one of Chelsea's last opportunities to cash in on their homegrown star who they value at £50million.
Should a fee be agreed with Atletico, he would still need convincing to leave, having never shown an eagerness to head for the exit of the club he joined aged six.
Diego Simeone is keen to snap up the England international, but it could cost up to £50m
Tottenham have also shown an interest alongside West Ham but currently, Atletico look like the likeliest to snap up the England international if they can agree personal terms.
Mail Sport asked Chelsea head coach Enzo Maresca for his stance on Gallagher in Atlanta on Tuesday and he said: ‘When the transfer window is open, unfortunately anything can happen. Not only for Conor, but for all of the players.’
Gallagher had been an integral part of Chelsea's team last season, having featured in all but one of their 38 Premier League matches.
The midfielder scored seven goals and provided nine assists in 50 appearances in all competitions for the Blues during the campaign.
Gallagher had captained Chelsea for large parts of the season due to the absence of Reece James due to injury.