The former Brazil international won two Premier Leagues, the FA Cup, the Carabao Cup and the Europa League with the Blues, and was named club player of the year twice.
"What a player," Cairney told the Footballer's Football Podcast. "He's the best player I've played with by a mile.
"His touch, his brain, the way he moves the ball, his decision making.
"I just can't imagine what he was like to play with seven or eight years ago at Chelsea.
"In my opinion he doesn't get the recognition he deserves because (Eden) Hazard was that good in the same team that people don't speak about him in the way that they should.
"Hazard was ridiculous but for four, five years Willian was one of the best players in the Premier League.
"He's 35 now and still when he stops dead and shifts it you can't get near him.
"What a player."