Mourinho became aware of the availability of the Spain forward on Monday and Chelsea signed him for £21million on Thursday, swooping swiftly for a player Manchester United had pursued for around a month.
The World Cup winner joined Baba Rahman, a left-back signed from Augsburg, in moving to Chelsea in the wake of the 3-0 loss at Manchester City.
Both could make their debuts in Sunday's match at West Brom as Chelsea bid to recover from their worst start to a season in 17 years.
"For me, it was one minute," said Mourinho, before recalling the conversation.
"'Is it true that you want to leave Barcelona?' 'Yes, it's true
I love it here
It's my home, but it's time for me to move.' 'Did you sign already for another club?' 'Almost, but not yet.' 'Do you want to come here?' 'Yes, I want.'
"I told people in the club 'it's over to you'
(But) I thought he would never leave Barcelona."
Manuel Pellegrini became the latest manager with a surname beginning with the letter 'P' to inflict defeat on Mourinho's Chelsea.
All the Blues' defeats since the start of the 2014-15 season came to managers with surnames beginning with P.
It is a quirk Mourinho can cope with, if another can be adopted.
"I wouldn't mind to go to the superstitious side of the game," Mourinho said.
"I never win against a P and seasons where I start with a 2-2 at home I win the treble.
"2-2 Porto-Belenenses, we won the treble (in 2004), 2-2 Inter-Bari, we won the treble (in 2010)
2-2 against Swansea?"
Chelsea followed the opening draw with Swansea with a heavy defeat to City and Mourinho is unhappy.
"We have one point from two matches
I'm not happy at all," said Mourinho, before listing those underperforming.
"I'm not happy with anyone
But, for me, the first thing is I'm not happy with my form, because I used to get better results than I'm getting (now).
"This is the start of everything."
Mourinho, who described himself as the Happy One on his return to Chelsea two summers ago, is eager for self-improvement as well as collective.
"I go to the gym every day now to get fitter," said Mourinho, who says he went once every six months previously.
"Maybe I need more action on the touchline, so I need to be fitter."
Further signings could follow Pedro and Rahman, if the chance arises before the September 1 deadline, but Mourinho refused to comment on Everton's John Stones or Paul Pogba of Juventus.
"People used to call it the transfer window
I sometimes call it the opportunity window," Mourinho added.
"While the window is open there is always an opportunity."
Source : PA
Source: PA