The 20-year-old Burkina Faso international, who made his Premier League debut last December, arrives after Polish international striker Arek Milik moved to Napoli.
Traore will be reunited with coach Peter Bosz under whom he played at Vitesse Arnhem from 2013-15 on loan from Chelsea. He scored 16 goals in 14 Dutch league appearances at Vitesse.
Traore would be available for Tuesday's Champions League playoff fixture against Russia's Rostov, the reports added.
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(Reporting by Mark Gleeson; Editing by Peter Rutherford)
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