An Ilkay Gundogan brace together with strikes from Joao Cancelo, Riyad Mahrez and Raheem Sterling saw the Baggies thumped 5-0 at The Hawthorns.
Pep Guardiola’s side were outstanding on the night but they were helped by some truly woeful Albion defending.
The result means the Baggies have now conceded a staggering 22 goals in their last five home games.
Next up for Allardyce’s side is back-to-back relegation six-pointers against Fulham (Saturday) and Sheffield United (Tuesday).
And with those games so important, the boss locked his players inside the dressing room for over an hour - making it abundantly clear to them all that they must improve.
“I’m sorry you have had to wait so long but I had more important things to do in the dressing room,” Allardyce said when arriving for his press conference.
“A few home truths needed to be said.
“I think it had to be done now because there is a short period of time before the next game and then a short period of time for the game after that.
“And these two games really do stand high on what we need to do and how we need to try and perform.
“I needed to get my disappointment and my feelings out the way immediately after the game - that’s why it took so long
“They (the players) needed to air some of their views - that’s fine because we are all in it together.
“But I was disappointed with that performance, to say the least.
“To get it out the way means it’s done and dusted.
“Hopefully, we will learn from it and we won’t see it again between now and the end of the season. I can accept defeat but not like that.”
Allardyce said 38 points is normally enough to keep you in the Premier League - meaning Albion need to take 27 points from their remaining 18 games.
But while that looks a tall order, the boss said his players must continue to believe they can do it.
“We have to believe - we have still got 18 games to go,” he continued.
“I know it’s a difficult task and we’ve got to achieve a minimum of 27 points from 18 games.
“That is the minimum to stay safe.
“We have to take it one game at a time. But what will determine our fate is what happens in the next two games.
“If we win those next two games we are going to get further up the table.
“Hopefully, we going to take three points off Fulham and three points off Sheffield United.
“That would ease the pressure on us and put the pressure on them. That is what we have to aim for.
“If we can’t achieve two victories then a win and a draw is the least we have to get.”