Etienne Capoue struck after just 32 seconds as Watford added to Leicester’s domestic woes by beating the Premier League champions 2-1 at Vicarage Road on Saturday.
Roberto Pereyra netted Watford’s second after 12 minutes and Riyad Mahrez’s penalty couldn’t revive Leicester.
Claudio Ranieri’s side are still looking for their first away win of the season and remain stuck within sight of the relegation zone.
Leicester will return to Champions League action against Club Brugge next week but while they have impressed in Europe, they continue to struggle for form back at home in the Premier League.
Once again Jamie Vardy failed to find the target, extending his club drought to 13 games although the forward did follow up his midweek strike for England by winning the penalty that allowed Mahrez to bring his side back into the game.
But the defensive weaknesses that have marked their disappointing title defence were again all too apparent as Watford claimed the victory that consolidated their place in the top half of the table.
Walter Mazzarri had been left incensed by Watford’s performance in the 6-1 defeat at Liverpool immediately before the international break and had insisted the anger shared by manager and players would ensure there would be no repeat.
But as anxious as the manager was to see his players recover quickly, their response was even more impressive than he might have hoped.
Just 32 seconds had elapsed when Watford exposed Leicester’s fragile defence to take the lead, and they had doubled that advantage within 12 minutes.
Pereyra was involved on both occasions, with the Argentinian’s left wing cross helped on by Troy Deeney’s backward header into the path of Capoue, who placed his volley beyond Leicester keeper Ron-Robert Zieler.