Kevin Trapp out until next year
The 29-year-old ex-Paris Saint-Germain shot stopper was injured colliding with team-mate Makoto Hasebe, who also suffered a head injury and is out for a week, in the 2-1 league victory at Union Berlin on Friday.
"Unfortunately, that is not good news for Kevin or us. We wish him a speedy recovery and are sure he will come back in top form," said director of sport Bruno Huebner.
Frankfurt's Danish reserve goalkeeper Frederik Ronnow will deputise, starting with Thursday's Europa League away game at Portuguese side Victoria Guimaraes after Frankfurt lost their opening match group match 3-0 at home to Arsenal.
Having spent three years with Eintracht before moving to Paris, Trapp returned to Frankfurt in 2018 after failing to hold down a regular first-team spot at PSG.
He has played in three friendlies for Germany since his 2017 debut and is currently third-choice behind captain Manuel Neuer and reserve Marc-Andre ter Stegen.