Irons midfielder Joe Cole had delivered a vintage display on his return to his boyhood club following a free transfer from Liverpool this week, rolling back the years as he set up a double for defender James Collins with superb crosses from the left flank in each half to turn the tie around after Tom Cleverley had fired United into the lead on 23 minutes.
However, as the clocked ticked into stoppage time, Dutchman Robin Van Persie - who had started on the bench - ghosted into the right side of the penalty area following a superb crossfield pass from fellow substitute Ryan Giggs, before drilling a low shot into the bottom corner to force a replay.
Allardyce said: "You have never beaten Manchester United until the final whistle blows and I have learned that in the past.
"When Matt Jarvis broke down the left-hand side late on and put in a great ball for Mark Noble to head down to Matt Taylor I was going, `please put it in', because Manchester United weren't going to come back from that.
"But if we didn't put it in (I knew) it would be nail-biting until the final whistle because somewhere along the line they might just carve out a chance with the quality that they have got."
Allardyce felt his team could take great credit from their efforts.
He said: "It was a great performance and in that dressing room we almost feel like we have lost when you get done in the last minute.
"But we are still in the hat, we have a difficult January now. That game gets fitted in, and we have some important league games before and after that, so we have to get the players fit who are out injured."
- Sports & Recreation
- Manchester United
- Tom Cleverley
- James Collins