Republic of Ireland under-21 midfielder Will Smallbone is out for the season after suffering an ACL injury against Leicester City last Saturday.
Smallbone started against the Foxes at the King Power Stadium and had impressed before colliding with Timothy Castagne around the hour mark.
He actually tried to play on but it quickly became apparent that the 21-year-old had done serious damage and he was substituted.
On Monday, Saints boss Ralph Hasenhuttl confirmed Smallbone is facing a lengthy spell out.
"We have after the last game another big injury with Will Smallbone," he said. "It is the worst for him. It's an ACL injury so half a year out, maybe. It's horrible for the young lad."
It's a terrible blow for a highly promising player.
Hasenhuttl had been complimentary about his display directly after the Leicester game, which Southampton lost 2-0, saying: "Will was good. He was fighting and had a few good moments, we had a few chances in the first half also.
"I like him more in the 10 position than the six position. He had some good solutions, he also had some good chances on Saturday - one good chance to score.
"He's a good footballer and learns to work against the ball also. Also, he was fighting very hard. It was a step forward in his development, definitely."
