Southampton's Tino Livramento faces a "long break out", manager Ralph Hasenhuttl said after the defender was taken off on a stretcher during the Saints' 2-2 draw with Brighton.
The 19-year-old was given oxygen as he left the pitch after an innocuous challenge with Enock Mwepu.
Hasenhuttl was unable to confirm exactly what injury he had suffered but admitted it looked "very serious".
"We are not sure yet, but it does not look good," he said.
"He's very frustrated because he knows it could be a long break out. He will come back stronger.