Leicester are no Barcelona, but Chelsea need to watch out for Foxes Messi - Riyad Mahrez in crunch F

Chelsea’s season has felt as if it has been unravelling for a while now. Defeat at Leicester City in this weekend’s FA Cup quarter-final will feel like the coup de grace.

It is pretty crude assessment, given what they were up against this week in the form of the sprite from another planet called Lionel Messi – but the Blues now find themselves in the same mode that has been the default setting for neighbours Arsenal in recent years: FA Cup or burst.

The top four (and a place in next season’s Champions League) is not out of the question, but would still feel deflating after last season’s heady and untroubled march to the title, when Antonio Conte was still on speaking terms with Diego Costa and Eden Hazard was untouchable.

The irony is that Chelsea were pretty good in spells at the Camp Nou on Wednesday night. Few teams enjoy as much possession as they did in that imposing cathedral, and in Willian, they had one of the most dangerous players on show. N’Golo Kante and Marcos Alonso also seemed to relish the challenge and looked world class.

It was just their misfortune, as it has been for so many other clubs with ambition, to come across the genius that is Messi at the peak of his powers – a player who somehow just seems to feed off the biggest occasions to touch the highest levels in his astonishing repertoire.

Where others might be cowed by expectation and responsibility, the Argentinian thrives.

Two goals, an assist, bursts of speed, feints and deftness of touch to take the breath away – as well as Chelsea hopes.

It really was a privilege to behold, even if it probably did not feel like it to the visiting fans perched at the top of the most vertiginous corner of the stadium.

There had been a narrative thread of optimism going into the two-leg tie that Messi had previously failed to find the net against the Blues in numerous attempts – infamously failing with a spot kick in that astonishing 2012 semi-final when the 10-men of Stamford Bridge defied the odds with an historic 2-2 draw.

But there was no subduing him this time.

It is very true that Chelsea did not help their cause by making so many individual errors. One from Andreas Christensen in the first leg was enough to allow Messi to plunder a vital away goal.

Here, Thibaut Courtois was at fault for not blocking a near post drive, while possession lost by Cesc Fabregas in a crucial area gave the King of Catalunya all the scope he needed to fashion goal number two for Ousmane Dembele. But has there ever been a more sublime exposer of errors than Messi? Has there been anyone with his relentless ability to make truly great players look ordinary?

“It’s a big disappointment tonight because we were courageous and together - but when you play against a Messi as effective as that, it’s that much more difficult,” said a resigned-looking Olivier Giroud after the 3-0 defeat which confirmed Chelsea’s exit from the competition.

“I don’t think the score reflected the way the game went, but in the big matches, he has the capacity to make the difference.”

There will be no Messi to worry about at the King Power Stadium, but it should be remembered that when Riyad Mahrez and Jamie Vardy went to town on the Blues during their march to that most unlikely of titles, it did for Jose Mourinho, who was axed immediately following a 2-1 defeat there.

With Conte’s future a source of constant speculation this season, another defeat would stoke up the rumour mill still further and leave us ever nearer the end game which seems the inevitable outcome of any dip in fortunes for the Blues, however successful a coach has been in previous campaigns.

The season can still end well, and in silverware. It is not as if the team looked ready to down tools in Barcelona. But raising your game to play on that stage is different from a Monday night at Watford, as we recently discovered.

“If we play with this kind of spirit, like we did in the Camp Nou we can hope for good things for the end of the season,” Giroud insisted.

We may be a little clearer about whether he is right, come Sunday night.