Fettis - who was voted the PFA Division 3 goalkeeper of the year last season - had to go off at half time after a challenge from Oxford forward Steve Basham left him with a knee injury.
Tigers boss Peter Taylor said: "Alan was definitely caught by Basham and it's disappointing none of the officials seemed to spot what happened.
Fettis Injury Worry
"We're hoping it's just bruising but it's too early to say at the moment. We'll know more as the week goes on but obviously it's a concern."
The Northern Irish international has yet to miss a game since rejoining from York in January, but Paul Musselwhite could now get chance to stake his claim for a starting place.
The 34-year-old is the only other professional goalkeeper at the club, and after keeping a clean sheet in the second half at the Kassam Stadium he may get to play in the Bank Holiday double header at the weekend; Cheltenham come to the KC Stadium on Saturday, before City go to Cambridge on Monday (both 3pm kick-offs).
Fans wishing to go to the Abbey Stadium must buy a ticket in advance. 1400 seat (£15 adults/£10 concessions) and 1000 terrace (£12/£8) tickets are on sale now.
The Northern Irish international has yet to miss a game since rejoining from York in January, but Paul Musselwhite could now get chance to stake his claim for a starting place.
The 34-year-old is the only other professional goalkeeper at the club, and after keeping a clean sheet in the second half at the Kassam Stadium he may get to play in the Bank Holiday double header at the weekend; Cheltenham come to the KC Stadium on Saturday, before City go to Cambridge on Monday (both 3pm kick-offs).
Fans wishing to go to the Abbey Stadium must buy a ticket in advance. 1400 seat (£15 adults/£10 concessions) and 1000 terrace (£12/£8) tickets are on sale now.