World Cup winner Fernandez arrived from Benfica on deadline day in a British-record £106.7million deal to cap more than £300m of spending by the Blues in January.
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Chelsea were forced to sweat over the 22-year-old midfielder by Benfica chief Rui Costa as talks went down to the wire.
Now, sources close to Blues co-owner Todd Boehly have revealed a frantic and chaotic conclusion to the transfer saga.
Benfica sent the final contract through with seven minutes to go until the deadline on Tuesday.
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However, the deal was threatened in the dying moments of the window because DocuSign would not load on Fernandez's phone.
The Argentine was unable to sign the paperwork until Blues owners Boehly and Behdad Eghbali directly intervened.
talkSPORT has been told the Chelsea chiefs were on FaceTime with Fernandez with minutes to go until the deadline - teaching him how to use DocuSign.
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Eventually, the club beat the deadline with 180 seconds remaining.
The Blues agreed to pay Benfica £30m up front, with six further instalments.
Fernandez signed an eight-and-a-half year deal at Stamford Bridge, in keeping with the club's policy of amortisation: spreading costs over numerous years to stay within Financial Fair Play rules.
"I am so grateful to Chelsea and its ownership for doing everything they could to make me a part of this project," Fernandez said.
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"I'm happy and excited to join the pride of London."
Fernandez became the eighth signing of a frantic window for the Blues, with a total January spend of around £318m.
Reacting to the Fernandez deal, Chelsea manager Graham Potter said: "Every transfer is a gamble. If you look at the market for midfield players - and midfield players that haven't won the World Cup - you'll be quoted a lot of money.
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"We've got a player with a huge personality; he played in midfield for Argentina who won the World Cup. He has attributes that can help him play in any league in the world.
"He has already played well in the Champions League and we followed him before the World Cup, we knew about him anyway. So we're confident about the personality."