Roy Hodgson, the former England boss, has been appointed as the new manager of the Hornets.
Ranieri was sacked on Monday after three-and-a-half months in charge with the Hornets 19th in the table.
The Englishman left Crystal Palace at the end of last season.
At the end of the current campaign, all parties will assess the future of the manager.
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He is 74 years old and has managed 17 different clubs in his 46-year career.
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Ranieri had failed to turn around the team's fortunes since he was appointed in October. The Hornets dropped into the bottom half of the table for the first time this season after Friday&s loss to Norwich.
They have not won in nine games in all competitions, their longest run without a victory since December.
It was the biggest margin of victory in the division since 1993, when they beat Everton by a single goal.
Since a 2-0 win vLiverpool in February 2020 in which they scored three goals,Watford have gone 30 games without a clean sheet, the second-longest such run in the history of the competition behind West Brom's 34 games in 2011.
Since the start of the season, only one team has picked up more red cards than the other.
After the loss to the Canaries, he was asked if he would continue as a manager. I want to keep fighting. I have never given up on my career.
They lost seven of their last eight league matches. Their most recent victory was against Manchester United.
Since the departure of Javi Gracia, five head coaches have been fired by the Hornets.
During that time period, the club has had six departures and two spells as caretakers.
The Pozzo family has had 15 permanent managers since they took over the Hornets.
The re-arranged game will take place on Saturday, February 5 at 6pm.
Sky sports reporter Gary Cotterill.
Roy Hodgson has all the experience in the world and is a safe pair of hands. This will be his 22nd job in 46 years. He worked miracles at Crystal Palace.
They lost their licence to thrill when he arrived. He did a similar job at Cottagers after they finished 11th in the Premier League.
He is a very experienced manager having also been at Inter Milan, and England. He wants to show he can operate at the top level even at his age.
He left Palace through his own decision, but as he told Sky Sports News last week, he still has a hunger and believes he can turn things around. Maybe he caught the ear of the owner of the Hornets.
He signed some paperwork on Monday and was already at the training ground.
Sky Sports pundit Paul Merson...
I think it is a good appointment. They need to stop leaking goals left, right and centre and steady the ship.
What Roy brings is experience. He doesn't mind playing 11 players at the back and the crowd booing as long as he gets a point. At the moment, they need that.
Some people might say they need someone younger who is going to come in and be flamboyant, but they have already had that, and they have been getting battered everywhere they go.
They have every chance of staying out of trouble now.
Ray Lewington was a manager atWatford Road almost 20 years ago.
Alan Lau, chair of the ESEA Hornets, says the experienced pair will need to get an instant reaction from the players if they are to stay in the league.
Roy Hodgson is a good fit for us. Lau told Sky Sports News that he has a good track record and has worked with the Pozzo family before.
Ray Lewington was in charge at Vicarage Road when we were in a more precarious situation than we are at the moment.
It is vital that Roy and Ray get a response from the players to give us the best chance of staying in the league.
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