Walker has made just four substitute appearances for the Hornets since arriving from Middlesbrough in August.
"His time on the pitch has suffered due to the form of other players in the squad," Watford manager Malky Mackay told the club website.
"Josh and I were both in agreement that several full 90 minutes' worth of League football is just what he needs right now.
"It's five weeks for him at a good standard. We both feel it will be very beneficial, with the view that he'll come back in January to once again challenge for a starting berth."
Walker, who can also play at centre-half, signed a two-year deal at Vicarage Road in the summer after making just five starts and eight substitute appearances during his time at the Riverside.
Source: BBC Sport
Source: BBC Sport