The Dutchman was hurt in his homeland as his side trained at the GelreDome on Wednesday ahead of Thursday’s Europa League qualifier against Vitesse Arnhem.
He sustained the injury towards the end of the session and left the stadium wearing a protective boot and using crutches.
He then travelled back to the team’s hotel in a van behind the coach carrying his squad and it is understood he has been advised he should have surgery as soon as possible.
Koeman will be in the dugout on Thursday as Southampton look to protect the 3-0 lead they built last week at St Mary’s.
But his assistant and brother Erwin Koeman will then attend the post-match press conference in his place.
The man who scored Barcelona’s winner in the 1992 European Cup final against Sampdoria was due to have an MRI scan in the hours before the match in Holland.
With a flight home after the game and another to Newcastle before Sunday’s game with Steve McClaren’s side at St James’ Park, Koeman may have to miss the fixture.