The 1. FSV Mainz 05 striker Conor Casey will be sidelined for the next six months after tearing a cruciate ligament.
A scan confirmed the extent of the injury at the weekend and Casey will remain in America for surgery, which is expected to take place this week.
UEFA Cup test
Casey was unable to hold down a regular place in Jürgen Klopp's side last season - completing just seven matches from 28 Bundesliga appearances - and has been linked with a move away from Mainz. He now misses the club's first-ever match in European competition, at home to Armenia's FC MIKA in the UEFA Cup first qualifying round on Thursday.