After joining from Napoli last summer, Sarri has received criticism from some sections of the Chelsea fanbase because of his tactical stubbornness and previous reluctance to use academy graduates Ruben Loftus-Cheek and Callum Hudson-Odoi.
But the Italian has guided the Blues to the EFL Cup final - which they lost on penalties to Manchester City - and the Europa League final - their first European final since 2013 – and, if they were to win beat Arsenal in Baku, Hasselbaink thinks this season would have been a success for Sarri.
"They had the League Cup final - that they lost to Man City on penalties - and the FA Cup, where they (reached the fifth round).
"If they win the Europa League final it's been a successful season for Chelsea."
It’s not been an easy debut season for Sarri at Chelsea.
During his first season in charge, Sarri has had to contend with Kepa Arrizabalaga’s refusal to be substituted in the EFL Cup final, an impending transfer ban for the next two transfer windows and increasing speculation surrounding Eden Hazard's possible departure to Real Madrid.
Despite the turmoil surrounding the club, Sarri has guided Chelsea to a third-place finish in the Premier League, qualifying them for the Champions League next year.
And Hasselbaink believes this is a perfect launchpad for them to regroup over the summer and improve next season.
"It's a start,” Hasselbaink said.
"From where they came from, it's a big improvement. Now they need to improve again for next year. They need to be title contenders.”
Sarri and Chelsea face Arsenal in the Europa League final in Baku on May 29.
It is still not clear, however, whether Sarri will remain at the helm at Chelsea this summer, with rumours linking him to the managerial opening at Roma.