Walker has been limited to few opportunities at Vicarage Road this season and will be at Broadhall Way until January 5.
Hornets manager Malky Mackay believes Walker will benefit from playing regular first-team experience.
"Josh and I were both in agreement that several full 90 minutes' worth of League football is just what he needs right now," he told the club's official website.
"His time on the pitch has suffered due to the form of other players in the squad, It's five weeks for him at a very good standard."
Source: PA
Source: PA