The Spain international has arrived at Stamford Bridge on loan for an initial fee of around £3m and an option to make the deal permanent for approximately £30m.
He arrives to bolster Chelsea’s midfield, which already contains Jorginho, Mateo Kovacic and N’Golo Kante – but the latter’s worrying injury record in recent months urged Thomas Tuchel to take action.
The 26-year-old was a key component of Atletico’s La Liga winning side last season, making 33 appearances.
But his form had gone awry in Spain and Diego Simeone was happy to see him leave to alleviate their wage bill, allowing them to sign Antoine Griezmann on loan from Barcelona in an extraordinary deal.
Nevertheless, if the Blues can get the best out of Saul then they have got themselves a fine player who can play in a number of positions across several tactical systems.
The transfer came about late after an agreement was finally reached on Deadline Day despite talks breaking down earlier in the window.
And the midfielder has stated how his fellow countryman Kepa helped convince him that west London would be the perfect location amid interest from Manchester United.
He said on Twitch: “I’ll be flying to London tomorrow, I’m coming to the champions of Europe.
“I’ll fight for my place in the starting 11, it’s important that [Kepa] is there.
“Kepa has been my friend for a long time. It’s important that he is there.
“I would have second thoughts if he wasn’t at Chelsea. [Cesar] Azpilicueta and Marcos Alonso also helped me join Chelsea.”
In a message to the fans, Saul said: “I am very excited to start this new challenge with Chelsea.
“Blues fans, I am one of you now and I can’t wait to wear the shirt, start training and see all of you. See you soon!”
Chelsea also had an offer rejected for Jules Kounde, who had been strongly linked with a move to the club throughout the summer.
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A verbal agreement was in place for a fee of around £52m but Sevilla demanded £13m extra at the last minute to leave the deal hanging by a thread.
The Blues failed to even contact the Spanish side after they moved the goalposts, deciding against a move.
Chelsea are relatively well-stocked at the back, with Cesar Azpilicueta, Thiago Silva, Andreas Christensen and Antonio Rudiger.
Trevoh Chalobah has also emerged as an option after a bright start to the season while Reece James can also play in a back three.