The 27-year-old forward was at his most effective against Cardiff City, scoring three times in a come-from-behind 4-1 win over the outclassed visitors.
Hazard's hat-trick boosted his tally to five goals in as many Premier League appearances and kept him on track to beat the 23 he netted in all competitions in his final season at Lille.
Sarri admitted he is now considering whether the FIFA World Cup's Silver Ball recipient has surpassed even Lionel Messi and Cristiano Ronaldo.
"I thought Hazard was one of the best players in Europe, but now I am changing my mind – maybe he is the best," Sarri said.
"I have spoken to him in the last two days, yesterday I told him for me he can score 40 goals.
"But [to do that he has to] spend less energy far from the goal and play in the last 25 metres."
Sarri's mood after the convincing victory was further improved with the news Mateo Kovacic's muscle problem, which forced him off early in the second half, had been deemed not to be serious.
Neil Warnock, who lost Harry Arter to a bout of dizziness at half-time, echoed Sarri's admiration for Hazard, but felt his Cardiff side could have made life tougher for the in-form hosts.
"Hazard is a good player, but that's what you're going to get when you're playing against these teams," Warnock said.
"I suppose as a neutral it's great to watch him perform like that. He's unplayable at times.
"We were in the game for 80 minutes and once again I'm really disappointed with the goals we conceded."