The Robins have yet to enjoy a league victory this season but have found their form in the cups with victories over Gillingham, Bristol Rovers and Crystal Palace.
But they travel to St Mary's without O'Connor who will have a scan on an injury on Wednesday, one which is expected to sideline him for months.
Louis Carey has a bug and will not feature while Aden Flint and Marvin Elliott are carrying knocks. Head coach Sean O'Driscoll is unlikely to make many unforced changes apart from Marlon Harewood and Elliot Parish, who have featured in their cup victories.
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