The Saints picked up a remarkable 4-3 win at Aston Villa, and were 4-0 up at one stage of the game.
But a late challenge from Villa's Trezeguet caused him to land awkwardly on his knee.
Hasenhuttl told reporters: "It doesn't look good. It was right in front of me and we know his history with knees.
"We hope it is not too bad an injury but we must have a look at the scan.
"He overstretched his knee which is always dangerous and he was immediately shouting ''My knee, my knee."
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