The Australia international limped off against Ipswich after turning an ankle and he is touch-and-go for the clash with Gianfranco Zola's Hornets.
Jon Stead popped up with a 90th-minute winner against his former club on Saturday after Steven Davies drew the strugglers level and the duo are competing to replace the injured Ryan Taylor in the starting line-up.
The former Rotherham man left the pitch after just 16 minutes with a fractured collarbone which will rule him out for at least six weeks.
If the specialists believe that a metal plate will need to be inserted in to his shoulder then the big striker could be ruled out for the rest of the campaign.
Albert Adomah remains on African Nations Cup duty with Ghana while the impressive Louis Carey is set to keep his place alongside Liam Fontaine at the heart of the City defence.