Borussia Dortmund sporting director Michael Zorc recognises Christian Pulisic is the next player they face a battle to keep hold of.
Pulisic, 19, is currently under contract until 2020 at the Westfalenstadion.
But having lost Ousmane Dembele last summer, and with Pierre-Emerick Aubameyang joining Arsenal, BVB are aware that the American international is now their biggest asset.
Zorc said: "We plan well beyond the summer with him."
BILD says Liverpool, Manchester United and Bayern Munich are all monitoring the American closely.