The former Sunderland striker has scored six times in 11 appearances since his January move, the most recent in the 3-0 win over Blackburn on Monday.
But the England international was forced to come off in that impressive victory and will be assessed ahead of the meeting with the promotion-chasing Hornets.
Former Plymouth striker Joe Mason scored following his introduction against Rovers and he is the likely replacement if Campbell is unavailable.
The midfield four of Jordon Mutch, Aron Gunnarsson, Craig Conway and Kim Bo-Kyung are likely to line up again after impressing in the comfortable victory.
Cardiff make the trip with an eight-point cushion over Gianfranco Zola`s men and seven ahead of second-place Hull.
- Sports & Recreation