The Austrian will be unable to call upon Takumi Minamino as he turned out for Liverpool in the tournament before his loan move to St Mary's, so he is ineligible for this fixture.
With Theo Walcott and Danny Ings also not expected to return from injury until after the international break, Che Adams - who has just been called up to the Scotland squad - is likely to be partnered by Nathan Redmond at the forefront of the attack.
Nathan Tella could also find himself back among the substitutes if Moussa Djenepo is selected to start on the left-hand side, while Stuart Armstrong should be given the nod on the right if Redmond is chosen to lead the line.
Mohammed Salisu started and kept a clean sheet against Wolverhampton Wanderers in the fifth round and could start alongside Jan Bednarek in a tournament where Southampton are yet to concede a goal this season.
Southampton possible starting lineup:
Forster; Walker-Peters, Salisu, Bednarek, Bertrand; Armstrong, Ward-Prowse, Diallo, Djenepo; Adams, Redmond