Kostic is a player West Ham are keen on this summer and they have already slapped in a bid for him as they aim to improve on the left side of the pitch.
The Serbian star won the Europa League last season with Eintracht Frankfurt, missing just one match in the competition, but otherwise playing every minute and assisting in the final.
West Ham are awaiting a response to their offer but whatever the result of that attempt they may have another headache to deal with.
Kostic is not fully sold on a move to West Ham and therefore the Hammers will need to convince the midfielder to sign for them.
The Serbian star may not want to give up on Champions League football next season, by virtue of winning the Europa League, and settle for the Europa Conference League with West Ham.
En route to the Europa League win, Eintracht Frankfurt defeated West Ham in the semi-finals and Kostic played every minute of the 3-1 win.
In total, Kostic made 43 appearances last season to the return of seven goals and 15 assists and it appears West Ham have some work on their hands if they want to secure his signature.
