Manager Maurizio Sarri said his Chelsea players reacted with “mental confusion” as they failed to respond to the predatory Jamie Vardy strike that earned Leicester victory at Stamford Bridge.
The Blues lost at home for the first time this season after Vardy’s first-time finish in the box from James Maddison’s pass in the 51st minute.
Chelsea had been dominant before the break and struck the bar through Eden Hazard following a Harry Maguire error – the best of a host of chances
But having gone behind, it was Sarri’s side who became sloppy as Leicester grew in strength having resisted heavy pressure.
The Foxes would have doubled their lead but for Cesar Azpilicueta’s sliding block to deny Vardy after Kepa Arrizabalaga spilled a cross, before the keeper saved well from Marc Albrighton.
Chasing a goal of their own, Chelsea struggled to break their opponents down before missing two key late chances.
In the 89th minute, Antonio Rudiger nodded just wide from a corner, before Marcos Alonso struck the post with only the keeper to beat two minutes later.
“We played well for 55 minutes,” Sarri told BBC Sport.
“I think that, of course, in some situations we were unlucky but we could have done something better.
“After the goal the reaction was not the right reaction. We reacted a little shocked and with mental confusion. We had to only keep playing like we did before the goal, there was time to recover.”
The Blues had not lost at home since 1 April, against Tottenham, and this defeat leaves them level on points with fifth-placed Arsenal, who beat Burnley 3-1 earlier on Saturday. They trail leaders Liverpool by 11 points.
For Leicester, the win moves them up to ninth in the table and is their first victory from 11 matches in London.
Reports before the game spoke not just of pressure on Leicester boss Claude Puel from the club’s owners but of an apparent falling-out with his players, too.
There was little sign of the team lacking any motivation or desire on the pitch on Saturday.
Resisting the intensity of Chelsea’s attacking intent is by no means a small feat. Puel’s men not only managed to achieve that, but also to build on it as they forced themselves slowly but surely into the game.
It was Vardy’s goal, scored on his 250th appearance for the club, that sealed victory, but it was a win earned by the whole team through constant running, daring in defence, and precision when it counted.
With Manchester City up next on 26 December, it had been suggested Leicester’s following match at home to Cardiff on 29 December might even decide the future of their French manager.
Instead, this impressive victory, a first at Stamford Bridge since 2000, must surely stand as evidence of Puel’s worth.
Chelsea however, will now face questions of their own.