The teenage midfielder is a target for Arsenal, with Bayern Munich and Borussia Dortmund are also interested.
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All three clubs had scouts posted to check on Kostov in Red Star's Europa League win at Malmo last night.
Terzic, speaking with the Mirror, was asked if he believes Kostov could handle a move to the Premier League.
He replied: "Of course but not just any team, he could play for one of the big six clubs."
On that offer, Terzic also revealed: "I can't talk about exactly who has called but we have already received an offer worth €20million (£17.45m) for him."
The Gunners plan to keep tabs on Kostov for the remainder of the season before making a move this summer - unless he is sold in the next week.