The 69-year-old has signed a two-year contract with the Hornets.
He inherits a team struggling after promotion to the top-flight.
The Vicarage Road outfit are sitting 14th with seven points from their first seven games.
Need a VPN? Or seeking to switch? Then support Tribal Football by clicking here to sign up with our official partners NordVPN. Not only will you receive the best VPN experience, but also great discounts by signing up via Tribal Football. Thanks for your support!