Having rejected the chance to pen a short-term extension with the Blues, and with his contract up in 2025, everything was set for Gallagher to enjoy a seamless transfer to the Wanda Metropolitano.
Atletico even posted a picture of Gallagher taking in his new surroundings on their official X account, as the midfielder had completed his medical ahead of his expected £34m transfer.
In a separate deal, Chelsea were working to poach Samu Omorodion from Diego Simeone's ranks, and Atletico were set to use the funds from the striker's sale to fund their Gallagher move after splashing out over £60m on Julian Alvarez.
However, talks between Chelsea and Omorodion collapsed due to a reported contractual issue, leaving Gallagher's future in limbo, and the midfielder was forced to return to the UK while all parties desperately tried to find a solution.
Maresca confirms Gallagher will not face Manchester City
While Omorodion will not be joining Chelsea this summer, the Gallagher deal is not completely dead in the water, and talks have been revived over the possibility of Joao Felix going back to Stamford Bridge.
In the meantime, Gallagher is training away from the first team and will not be involved in Sunday's Premier League opener against Manchester City, Maresca confirmed to the press, while also expressing hope that a solution will soon be found.
"Conor is back and training a bit apart, he is doing some medical tests and training, he won't be involved in this game," BBC Sport quotes the former Leicester City boss as saying.
"You know better than me that he is trying to find a solution with the club and we will see what happens. Hopefully Conor's situation finishes with the player, the club and all the people happy."
The Gallagher situation was recently labelled "a pity" by former Chelsea midfielder Emmanuel Petit, who also took aim at the Blues' scattergun transfer strategy as Maresca tries to manage an incredibly bloated squad.
Gallagher one of three players working away from Chelsea first team
No fewer than 43 players are in Chelsea's first-team ranks at the minute, although Trevoh Chalobah and Romelu Lukaku have also been banished from the main group and are training individually alongside Gallagher.
However, Lukaku's exit may only precede the arrival of another striker in Napoli's Victor Osimhen, but Maresca has insisted that he does not have time to worry about each and every player in his squad.
"We've been in America with 28 or 29 players so it has been quite good managing the squad, the other players are training apart," the 44-year-old added.
"I just finished today's session and I am thinking about the next session. If I start to think about 43 players, managing their time, I think it is not worth it."
No Chelsea players are carrying injuries ahead of Sunday's Premier League opener against Manchester City, although captain Reece James - who often handed the armband over to Gallagher last season - is suspended.
Meanwhile, the champions must cope without midfield fulcrum Rodri, and their record without the Spaniard in the Premier League makes for grim reading for the City faithful.