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.
Crisis? What crisis?
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.
Bad misses or bad luck?
This was not a vintage Chelsea performance – not the kind of showing we have come to expect from them at home since Sarri’s arrival in the summer.
Looking back on the match, the Italian will identify three big misses that cost his side.
There was Hazard, smashing against the bar in search of his 100th Chelsea goal, there was Rudiger’s header wide under pressure but with the goal gaping at a corner, and finally Alonso’s 91st-minute miss when clean through.
The Spaniard, played in behind the Leicester defence, side-footed his shot against the post and out, having been given what seemed an age to pick his spot.
Perhaps these were moments of bad luck. Sarri is renowned as being a very superstitious man, but he will not have failed to observe the clear dip in intensity from his side after going behind, which ultimately defined their afternoon.
They seemed shell-shocked after Vardy’s powerful strike, and only managed to conjure something of their finer play in the final few dramatic minutes. It turned out to be too little, too late.
Hazard left the field with a wry smile having missed out on the chance of becoming just the 10th Chelsea player to reach that landmark total of a century of goals.
Had he done so, the afternoon might have ended very differently for Puel, the man who gave him his senior debut 11 years ago for French club Lille. BBC Sport