The striker has spent the season on-loan with Southampton.
Speaking to Prime Slots, Hasselbaink said: "You can see that he is a very strong boy, very athletic and quick.
"He has a good eye for a goal and is single-minded.
"He is still learning his game and needs to improve but he is getting it right. If I was Chelsea, I would not sell him yet and I would see how he develops next season. Maybe he will prove to be a player that is good enough for the Chelsea team."
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