The 21-year-old Wolves player limped off after just 18 minutes of the 5-2 win over Cheltenham on Wednesday afternoon.
Vokes suffered a hamstring tear on the opening day of the season and has yet to recover from the injury.
"It`s been a nightmare loan for Sam and for us because we know how good a player he can be," City boss Keith Millen told the club`s official website.
"There`s clearly something wrong there and he needs to get himself fully fit again. He`s on loan until January 5 and it`s unlikely he will be fit enough for the first-team before then."
Source: PA
Source: PA