The defender was forced off late on in Saturday's impressive 2-1 win over Leicester nursing a hamstring injury and will be assessed as late as possible.
Hall aside, Gianfranco Zola is unlikely to make many, if any, changes to his starting XI following the victory, which came after back-to-back defeats for the Hornets.
Zola has no other new injury or suspension concerns, with captain John Eustace (back), fellow midfielder Prince Buaben (hamstring) and defenders Piccoli Neuton (ankle) and Nyron Nosworthy (hamstring) still sidelined.
Millwall make the trip across London level on points with Watford in the table but on a seven-match unbeaten run which includes successive wins over Huddersfield and Nottingham Forest, during which the Lions have scored eight times.
Alan Dunne returns to the Millwall squad.
The defender missed Saturday's visit to Nottingham Forest through suspension after picking up a fifth booking of the season.
But he may struggle to regain his place after the Lions ran out impressive 4-1 winners at the City Ground, with Tottenham loanee Adam Smith deputising well at right-back.
Danny Shittu was the only player to suffer an injury at Forest when he had a tooth knocked out but the big centre-half should be fine to continue.
Fellow defenders Jack Smith (groin) and Karleigh Osborne (calf) are also fit after recent lay-offs, so Paul Robinson and John Marquis are the only ones in the treatment room.
Skipper Robinson is recovering from a pelvic problem and striker Marquis is building up his fitness after an adductor muscle operation.