St Kilda captain Nick Riewoldt injured his right knee during the first quarter of the Saints AFL clash against Melbourne at the MCG on Saturday.
Riewoldt was on a long lead when the ball dropped short and he was forced to reach out to complete the mark.
His right knee appeared to hyperextend underneath his weight and he was immediately taken from the field at the 15-minute mark of the opening quarter.
When he re-entered the playing arena early in the second quarter, the champion Saint was on crutches and was subbed out of the game.
Any long-term injury would be a massive blow to the Saints who still hold hopes of making the finals.
At halftime, St Kilda led Melbourne by 16 points.