Matheus Pereira and Callum Robinson score twice as struggling West Brom overwhelm 10-man Chelsea and end the 14-match unbeaten start to Thomas Tuchel’s tenure.
The Baggies were as brilliant as the Blues, who had Thiago Silva sent off in the 29th minute, were abject.
The result moves the 19th-placed side to within seven points of 17th-placed Newcastle, and even if they do not manage the improbable and escape relegation, they will look back on this day with great pride.
Tuchel said he “did not see the second half coming”. His side took the lead through Christian Pulisic in the 27th minute, but then lost Thiago and conceded twice in first-half stoppage time before West Brom inflicted further damage after the break.
“There is no need to concede five goals when you have our quality,” he told BBC Sport. “We are all in on that – myself included.
“We could not adapt to the situation, which is a surprise as we were leading. We are normally confident enough to defend, but we were sloppy and kept making big mistakes. It’s tough to digest.
“I did not see that second half coming. We tried to give a bit of security with a back three, but we were never in a flow. We had some chances but we didn’t defend well.”
Two minutes after Pulisic was quickest to react when Marcos Alonso’s free-kick came off the post, Thiago was shown a second yellow for a rash challenge on Okay Yokuslu.
That was followed by a horror show in first-half stoppage time as Pereira took centre stage. The Brazilian’s first effort was a cute finish over Edouard Mendy before he finished off a gorgeous move with a low strike between three defenders. In between those goals, Matt Phillips produced a strike that came off the woodwork.
West Brom then went for the jugular after the break as substitute Robinson made it 3-1 with a sensational side-foot volley before Mbaye Diagne finished off a sweet move with an even sweeter strike.
England midfielder Mason Mount, brought on at half-time, tapped in to reduce the deficit but the Baggies resisted other Chelsea threats before Robinson capped off a great day for the visitors with a clipped finish.
“I am delighted – not only with the win, but the style,” Allardyce said.
“It has been coming for a while, but our lack of finishing has cost us. What we saw today is what we have been missing. The quality of our goals and our football brought us a fantastic result. We were desperate for it.
“We started really well, we pressed and didn’t sit back. I had three weeks to think about this game and sort the tactics. We said, ‘Let’s go, let’s go challenge them’ and it worked extremely well.”
Sluggish Chelsea severely punished
His reasons for leaving some first-team regulars from the first XI, including Mount and Kai Havertz, was because he wanted the freshest legs on the pitch because of the extra exertions of his benched players during the international break.
Those who started might have been sharper physically, but they were mentally sluggish at times with Thiago paying the biggest price.
It was a risk to start the 36-year-old Brazilian, who had just returned from two months out with an injury. He took ‘one for the team’ when he pulled back Pereira on the edge of the area, but his second booking was inexcusable for a player of his stature.
Dane Andreas Christensen, who had deputised for Thiago, came on as midfielder Hakim Ziyech made way.
Striker Timo Werner came into the match short of confidence and having missed a ‘sitter’ for Germany against North Macedonia. He probably wished he reacted as quickly as Pulisic did for the Blues goal, although he did set up Mount’s goal.
By that stage, both he and his team-mates were probably wishing they were still on international duty.
A side who had conceded only two goals in Tuchel’s previous 14 games were outwitted and embarrassed by one who had struggled to score this season.
Chelsea have four days to recover before they face Porto in the Champions League quarter-finals.