The versatile Mohsni, who - along with Ryan Hall - flanked striker Liam Dickinson in last weekend`s 2-0 victory at Macclesfield, picked up his fifth booking of the season late on and now serves an automatic one-match suspension.
Sturrock will assess his options in training before making a decision on Friday as to who will step in, with Anthony Grant, Jemal Johnson, Ryan Leonard and Neil Harris all leading the way.
Defender Luke Prosser (concussion) and midfielder Lee Sawyer (thigh) will again compete for inclusion as the Shrimpers, top of League Two and on a nine-match unbeaten sequence, aim to inflict defeat on an Oxford side whose run of one defeat in 10 has taken them up to fourth place.
Damian Batt returns from suspension for Oxford. Defender Batt missed the victory over Port Vale last weekend after he was booked for the fifth time this season, but is available for manager Chris Wilder for the trip to the table-topping Shrimpers.
Meanwhile, striker Jonathan Franks is set to be part of the matchday squad after fully recovering from a dislocated shoulder suffered in a training match in September.
The on-loan forward had gone back to parent club Middlesbrough for treatment but is now fit and will be fighting for a place alongside James Constable up front with Deane Smalley and Danny Philliskirk.
Rob Hall was recalled by West Ham during the week, leaving an opening in attack for one of the trio. The Us are still without injured quartet Tony Capaldi (back), Tom Craddock (knee), Anthony Tonkin (hamstring) and Andy Haworth (ankle).