Dernis sprained his right ankle in a challenge during the second half of Saturday´s 2-1 defeat to OM, and is set to be sidelined for around a month.
A statement on St Etienne´s official website, www.asse.fr, said the player should return to training by August 25, meaning he is likely to miss the team´s first five games of the Ligue 1 campaign.
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