Saints have confirmed that Luke Shaw has badly gashed his right knee.
The young left back was stretchered off during the win at Aston Villa after a heavy challenge.
Nigel Adkins was unable to give any timescale for a return but Shaw may face a short spell on the sidelines as a result.
Danny Fox could well be set for a spell in the team to deputise.
Gaston Ramirez was also forced off at Villa Park but that was simply cramp.