Arsenal loanee Lansbury and fellow midfielder Harley missed last weekend's win over Plymouth through suspension.
Ross Jenkins and Stephen McGinn deputised against Argyle, the latter making his full debut, and boss Malky Mackay must decide whether to change a winning line-up.
The depleted Hornets were also missing full-back Lloyd Doyley and winger Will Buckley due to hamstring injuries and they will be assessed closer to kick-off.
Centre-half Martin Taylor shrugged off a knock in time to make the bench and he will push for a start at the Walkers Stadium.
On-loan midfielder Tom Cleverley has gone back to Manchester United after his season was ended by a knee-ligament injury.