Frank Lampard warned Chelsea to learn a "big lesson" from the mistakes they made before Tammy Abraham's last-gasp strike rescued a thrilling 3-3 draw against West Bromwich Albion on Saturday.
Lampard's expensively assembled side were torn apart in the first half at the Hawthorns as Callum Robinson's brace and Kyle Bartley's goal left Chelsea 3-0 down.
Chelsea captain Thiago Silva was guilty of an woeful blunder on his Premier League debut when the former Paris Saint-Germain defender gifted the ball to Robinson for his second goal.
But Mason Mount reduced the deficit soon after the break and Callum Hudson-Odoi got Chelsea's second before Abraham equalised deep into stoppage-time.
It was a much-needed show of spirit from Chelsea after their defeat against Liverpool last weekend.
The prospect of a second successive loss had Lampard visibly fuming on the touchline in the first half, but Chelsea responded with a much-improved display.
While Lampard will be relieved by evidence of Chelsea's attacking potency, there must be questions about their shambolic defence and he admitted they need to improve significantly.
"You can't legislate for the mistakes that there were today. You can have as many meetings as you want but with those mistakes you give yourself a mountain to climb," Lampard said.
"The question was how much character would we come out with second half. I always felt we could do it, but it is still two points lost.
"We lacked a bit of urgency in the team. It is a big lesson for us."
'Pure mistakes'
Despite dropping error-prone Kepa Arrizabalaga after the goalkeeper's costly mistakes against Brighton and Liverpool this season, Lampard learned that Chelsea's problems run deeper than who is between the posts.
Lampard's £200 million ($254 million) close-season spending spree was supposed to turn Chelsea into title challengers, but on this evidence they are still way behind champions Liverpool.
"I'm not going to criticise every part of our game. There was nothing tactically you can analyse. There were just pure mistakes," Lampard said.
"I expect to see an improvement in our performances. But in terms of team fitness, it won't be until after the international break that I expect to see us up to our real level."
Willy Caballero took over in goal, but he was hardly convincing and Senegal keeper Edouard Mendy, signed from Rennes earlier this week, looks certain to make his debut soon.
It took just four minutes for Albion to exploit Chelsea's frailties.
Marcos Alonso is likely to lose his place at left-back when summer signing Ben Chilwell is fully fit and the Spaniard did his cause little good with a woeful header straight to Matheus Pereira.
Robinson was picked out by Pereira and he advanced to the edge of the Chelsea area before drilling a low shot through Reece James' legs and past Caballero's slow dive.
Silva made a horrendous mistake that led to Robinson's second goal in the 25th minute.
Allowing Mateo Kovacic's pass to slip under his foot inside his own half, Silva surrendered possession to Robinson with embarrassing ease and was left with a sheepish look on his face as the Albion striker made him pay with a clinical finish.
There was worse to come for Chelsea three minutes later when their sloppy defence failed to pick up Darnell Furlong at Pereira's corner and his flick reached Bartley, who was played onside by James and slotted past Caballero from close-range.
Lampard changed the tone of the match when he sent on Cesar Azpilicueta and Hudson-Odoi at the interval.
If Lampard read the riot act to his flops at the break, he got an immediate response as Mount fired home from 20 yards in the 55th minute.
Chelsea were more dynamic now and Hudson-Odoi gave them hope when he played a neat one-two with Kai Havertz and slotted into the far corner after 70 minutes.
With full-time just seconds away, Abraham came to Chelsea's rescue when the striker tapped in after Albion keeper Sam Johnstone turned Mount's shot into his path.