Frank Lampard said Chelsea's capitulation to lose 3-2 at relegation threatened West Ham on Wednesday is a sign of how much work he still has to do to make the Blues title contenders once more.
Chelsea were on course to move third in the Premier League when Willian's penalty gave them the lead in the first half at the London Stadium.
But the visitors did not learn from the reprieve they received from VAR when Tomas Soucek had a goal ruled out for offside as the Czech rose highest to level from another corner.
Michail Antonio then put West Ham in front early in the second half before Willian's free-kick brought Chelsea level once more.
Lampard's men then looked the more likely to claim all three points in the closing stages, but were caught on the counter-attack when Andriy Yarmolenko fired home a huge winner for West Ham's chances of avoiding the drop.
A 10th league defeat of the season leaves Chelsea just two points clear of Manchester United and Wolves in the race for a place in the top four and next season's Champions League.
"I wouldn't say it is the story of our season because it's been a good season, but there have been so many of these moments," said Lampard, whose side had won all three of their previous matches since returning from English football's three-month shutdown due to coronavirus.
"The players in those moments have to show an extra mentality to see a game through like this when you have a lot of domination."
After a 2-1 victory over Manchester City last week that sealed the title for Liverpool and the signings of Timo Werner and Hakim Ziyech, Chelsea have been touted as title contenders to City and Liverpool next season.
However, Lampard believes his side have to be far more consistent than they have shown during his first season in charge.
"I've seen us play great games, but I also have seen opportunities to close a gap or move into third wasted. That's the difference at the minute and shows how much work we've got to do."
Just a second win in 12 games takes David Moyes's men up to 16th, three points clear of the relegation zone.
A much-needed victory for the former Manchester United manager was all the sweeter after feeling the injustice of a VAR call for the second time in eight days.
Moyes was unhappy when Tottenham's opener in a 2-0 defeat last week was allowed to stand despite a handball in the build-up.
"Again the VAR stuff, I'm beginning to lose faith with it," said Moyes.
West Ham's win was the first for any side in the bottom five in 30 games since the Premier League's restart.
"We'd be lying if we said we weren't looking at the results (of the other sides)," added Moyes. "I didn't think we had been as bad as our results suggested."
The controversial VAR call came at 0-0 when Antonio was adjudged to be interfering with goalkeeper Kepa Arrizabalaga's line of vision from an offside position on the floor when Soucek swept the ball home from a corner.
"I really dislike VAR," said Antonio. "It's gone against us so many times. Today it didn't stop us, we keep going and we got the victory."
Moments later it got even worse for the hosts as Christian Pulisic was chopped down by Issa Diop and Willian confidently dispatched the spot-kick.
West Ham were level before the break, though, as Soucek rose highest at another Jarrod Bowen corner to power home a downward header.
Moyes's men were then rewarded for a positive start to the second half as Bowen's low cross was thrashed home by Antonio.
Pulisic remained Chelsea's most dangerous player and when the American was brought down again just outside the area this time, Willian produced another moment of set-piece magic.
But against the run of play Chelsea were punished for some lazy defending by Marcos Alonso as he did not track Yarmolenko's surge from inside his own half and after being picked out by Antonio, the Ukrainian cut inside onto his favoured left and fired past Arrizabalaga.