The 20-year-old joined the side on loan in January 2017 before the deal was made permanent four months later.
The midfielder, making his 13th appearance across all competitions this season, levelled matter for Michel Preud'homme’s men in the 39th minute after Ever Banega had scored the opener.
Franco Vazquez then scored twice within nine minutes with Franco Vazquez also adding his name on the scoresheet before Banega completed the rout with his brace.
Djenepo was replaced in the 62nd minute and Nigeria international Uche Agbo was introduced for Milos Kosanovic with 12 minutes left to play.
The loss leaves Standard Liege at the base of Group J with zero points and they take on Akhisarspor in their next Europa League clash on October 4.