The 29-year-old was told at the end of last season he wasn't in Antonio Conte's plans going forward, which was soon followed by the arrival of Alvaro Morata.
It meant Costa needed to find a new club, but frustratingly, the one club he wanted to join were in the middle of a transfer ban.
But on September 21, 2017, Chelsea and Atletico agreed on a £57 million fee that would see Costa return to Spain on January 1, 2018, when the transfer window reopened.
Speaking at his unveiling at the Wanda Metropolitano, Costa, who will wear the No.18 shirt, expressed his delight at being offered a return to the club where he made a name for himself.
"I thank the club for the effort they've made to bring me back," he said. "I thank them from the bottom of my heart.
"My fitness? You can see I'm looking much better now. I've prepared well for this and I'm looking forward to playing and helping the team.
"I've waited a long time for this. Now I have my mind set on playing. As much as I've trained, it's not like playing a match and I'm tired of training so much."
Costa is raring to go in his second spell for Atletico, although he probably didn't expect to be playing in the Europa League upon his return.
Diego Simeone's side finished third in Group C of the Champions League, meaning they will spend the second half of the season in Europe's secondary competition.
And it would seem even UEFA forgot that Atletico failed to reach the last 16 judging by the tweet they posted and soon deleted on the Champions League's account today.
In the screenshots below, UEFA awkwardly asked their followers how many goals Costa would score for Atletico in 2018.
Once they realised the error, UEFA deleted the tweet from the Champions League's account and reposted it under the Europe League's account. Check it out.
UEFA MADE AWKWARD ERROR
How awkward is that?
Worse still is that while Costa is being reminded of playing in the Europa League this season, his old club, Chelsea, are through to the last 16 having finished above Atletico.
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