Sarri wanted to introduce a new style of play to the Blues, but he never got the full backing of the fans and media.
But Willian does not think the Italian's one year reign was a disaster, as he was complimentary of the now Juventus coach.
"Sarri is a good coach," he told ESPN Brazil. "I can't tell you what happened.
"He had an interesting tactical idea, he liked to play a beautiful game, with the ball.
"Anyway, he won a title, the Europa League, and third place in the Premier League. I don't know why it wasn't better. He was a very calm guy on a daily basis."