The club's medical team are waiting for the swelling to reduce before further assessment and a definitive diagnosis and plan is made.
Dreher made his debut first team appearance in the win over AFC Bournemouth on the final day of the 2018/19 season, and joined the first team squad on pre-season tour in Switzerland - featuring in both Uhrencup fixtures against FC Luzern and BSC Young Boys. The midfielder also featured in the pre-season fixtures with Barnet, Bristol City, AFC Wimbledon, Hertha Berlin.
An update on Dreher's injury will follow in due course.