Manchester City - inspired by Carlos Tevez - recorded a 4-1 win and extended Rovers' run without a Premier League victory to nine matches, which leaves them just three points above the relegation zone, having played more matches than all the teams around them.
It was the worst possible preparation for Thursday's Carling Cup first-leg showdown with Aston Villa at Ewood Park.
So bad, in fact, that Allardyce is sharpening his axe.
"It is no excuse that we had the semi-final on Thursday," he said.
"Playing well in one game guarantees you a place in the next one.
"Not playing well puts your semi-final place under threat and that was a bad night.
"Thursday is a great game, it is a massive opportunity to get to Wembley and some of the players have jeopardised their position to play in the best game they have had for many years."
The problem for Allardyce is that his fringe players hardly covered themselves in glory at Villa in the FA Cup.
Only Jason Roberts (knee) and Benni McCarthy (back), who is angling after a January move, missed out through injury last night, which leaves Allardyce with a major selection headache knowing a positive result is essential to take into next week's second leg at Villa Park.
"The disappointing thing is that I wanted a hard job picking a team?
"Unfortunately the team hasn't played to its best and it has left me opportunities to make changes wherever I feel like it."
Having come so close to reaching Wembley, Allardyce does not want to give up the opportunity now.
However, he is also acutely aware of the dangers lying in wait if Rovers get sucked into a prolonged scrap for survival.
While he could have no complaints at last night's outcome given the gulf in class between the two sides, it just highlights the disappointments Blackburn have suffered in recent times.
"It is a big concern," he said. "That is five draws and four defeats now when, so easily, three of those draws could have been victories.
"That is the killer for us. We are running perilously close to the bottom three.
"It is not where we want to be and it is not where we should be.
"Unfortunately, 21 games in, we are."
Blackburn v Aston Villa Win to Nil: Blackburn 4/1
Source: Team Talk
Source: Team Talk