The Italian has also been criticised by fans and analysts for playing N'Golo Kante, one of the world's leading holding midfielders, out of position in order to accommodate Italian Jorginho as his deep-lying playmaker.
"I think the criticism is a little unfair towards him, he had a really good start to the season," Hasselbaink, who played for Chelsea from 2000-2004, told Sky Sports News.
"You need to give him the time, it's only six or seven months that he's been in the building, so you need to give him the time and you need to give him the trust.
"He's put Kante in a different position because he sees that position in a different way and that's where it (the criticism) started. Whatever you say, Kante's the best defensive midfield player at winning the ball back, so that has raised eyebrows."
Sarri, who has also deployed playmaker Eden Hazard as a withdrawn forward in some matches before the arrival of striker Gonzalo Higuain, has previously said he is determined to make his approach work.
Former Netherlands international Hasselbaink backed Sarri to eventually get things right and deliver success.
"He believes in playing football in a certain way and that is dominating the ball in possession," the 46-year-old added.
"That's how he believes Chelsea is eventually going to win titles and cups."
Chelsea take on defending champions and league leaders Manchester City on Sunday.
(This story corrects Jorginho's nationality in third para.)
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Amlan Chakraborty)
