Matheus Pereira and Callum Robinson scored twice as struggling West Brom overwhelmed 10-man Chelsea and emphatically ended the 14-match unbeaten start to Thomas Tuchel’s tenure at Stamford Bridge.
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.”
