Mourinho could barely conceal his satisfaction when asked how confident he was of pipping Spurs to the player.
"That's a good question because I think he already made his decision," he said, before nodding like the cat that got the cream when asked if that decision was Chelsea.
Villas-Boas had previously failed to sign Willian for Chelsea and Tottenham while the player had been at Shakhtar Donetsk, who eventually sold him to mega-rich Anzhi at the end of the January transfer window.
The Russian side's decision to rein in their spending meant he was put up for sale, with Liverpool the first to make a move.
Spurs followed but it was the intervention of Roman Abramovich that seemingly proved decisive, the Chelsea owner personally calling Anzhi counterpart and fellow oligarch Suleyman Kerimov to agree a fee.
The lure of Champions League football and the Mourinho factor were enough to convince Willian himself to enter into talks with a third Premier League club in less than a week.
Mourinho suggested Chelsea's attempt to sign him had begun before the Spurs medical, claiming it was already under way when he held a press conference on Tuesday before Wednesday's game against Aston Villa.
"Before Villa, I told you we have plan B and we have plan C so I was letting the door open to other possibilities," he said.
Tottenham declined to comment on Mourinho's jibes or Chelsea's recruitment tactics but Villas-Boas will be anything but happy at losing out to his former mentor.
Willian's arrival was also thought to be the trigger for Gareth Bale's world-record £93 million move to Real Madrid, although the winger is thought to be confident that it will still go ahead in the coming days.
It may have to wait for Spurs to decide on their own plan B to Willian, although signing Roma forward Erik Lamela may help convince them the time is right to let their talisman go.
If their pride can stand it, they could even try to sign whichever player Mourinho decides to let go once Willian completes his move.
"Do we need to sell? No, we don't," Mourinho said. "Do we need to loan? Probably yes, because I don't like to work with big squads."
Willian's arrival certainly places another huge question mark over the future of Juan Mata, who has been Chelsea's best player since joining them two years ago but has played just 65 minutes in two matches this season.
He was also Villas-Boas' first big signing for Chelsea, who again saw off interest from Tottenham.
But Mourinho said: "He's a player we all like, he's a player we all want to keep."
Mourinho's revelation also means that should Chelsea succeed in signing a striker before the window closes, one of Fernando Torres, Demba Ba and Romelu Lukaku will be sacrificed.
