Chelsea manager Antonio Conte has had a hard job of calming striker Diego Costa after a bust up last week.
The striker was linked with a big money move to China after he clashed with the boss – but Hasselbaink has urged Chelsea to keep hold of him.
Speaking on BBC Football Focus, Hasselbaink, who scored 87 goals in his four-season spell at Chelsea, emphasised how significant it was for Conte to get the best out of their forward.
He said: “The Costa effect. If you look at their body language, both of them, they were fighting a little bit, on an off.
Hasselbaink has praised the Chelsea boss for the way he has handled Costa
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“Conte has persuaded him in a way. ‘No, it’s my way’, and Costa starting playing better.
“I think Costa is very important for Chelsea and the odd game you won’t miss him, but when you play 10 games in a row, you will miss him.
“Chelsea can’t afford to let Costa go at this time of the season, and I think how calm Conte has done that is credit to Conte.”
Boss Conte has urged supporters to put their faith back in the fiery Brazil-born striker again after his dispute with the forward.
Costa is poised to return for the Premier League leaders against Hull City on Sunday (4.30pm) after he was dropped against Leicester last week following the training-ground bust-up.
Chelsea are currently top of the Premier League seven points ahead of second place
Chelsea train ahead of Hull clash
Fri, January 20, 2017
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Diego Costa was all smiles as Chelsea train before their clash with Hull
Chelsea can’t afford to let Costa go at this time of the season
Jimmy Floyd Hasselbaink
The striker was then swiftly linked with a move to Chinese Super League club Tianjin Quanjian who were thought to have offered the star £30million-a-year.
Costa trained with the squad from Tuesday onwards this week and is now set to make his 100th appearance of the club.
Conte said: “Diego's focus and concentration is at the level I want. He is mentally ready to play.
“I can put my hands in the fire. What does it mean? In Italy, if you put your hands in the fire, it means you are certain 100 per cent. I means you trust the situation 100 per cent.
“I’m sure that, if Diego starts, he will put all of himself into it for this club, the shirt and the fans, and do everything he can to win.”
Diego Costa was linked in a big money move to China last week
Hasselbaink continued with his praise of the 47-year-old Chelsea boss for the way he managed the club when he took over at the start of the season.
He said: “First of all, Conte, magnificent. In the line of Mourinho, Ancelotti, and all them lot, magnificent.
“Yeah in the beginning of the season he had it difficult, changed it towards his way, gave them the opportunity to play in the four.
“Changed it to a three where he is very comfortable. You see in his body language as well that he is more comfortable with that system.”