Manchester United manager Louis van Gaal may have been in defiant mood ahead of his side's FA Cup fourth round tie at Derby County on Friday but a defeat by the second-tier club could push the Dutchman to breaking point.

Van Gaal was booed by his own fans following Southampton's 1-0 win at Old Trafford last week which left United five points off the Champions League places.

Failure to qualify for a place among Europe's elite would be a huge blow but some of the pain could be offset by a good Cup run.

Indeed United's victory over Everton in the 1995 FA Cup final is now widely regarded as marking a turning point in the career of celebrated manager Alex Ferguson, whose early tears at Old Trafford offered no hint of the sustained run of trophy-laden success that was to come.

United executive vice-chairman Ed Woodward had been rock solid so far in his support of van Gaal, which is perhaps hardly surprising since he appointed him but losing to Midlands side Derby would be something else.

"It is fantastic of course, but that also gives you a lot of pressure," said van Gaal of Woodward's support at a news conference on Thursday.

"When the board has such confidence, the pressure is much higher than when they say it is your last game or something like that.

"When your confidence is not so high, you can fight against it. When you can fight, you have bigger motivation than when they support you.

"But I am always fighting."

Such was the heated atmosphere surrounding Old Trafford after the Southampton loss that reports subsequently emerged of van Gaal offering to resign.

But he dismissed those suggestions completely on Thursday, with van Gaal saying: "I have not mentioned that, ever," he said.

"This is the third time that I have been 'sacked' and I am still sitting here for you. I have always faced this (scrutiny)."

Van Gaal also appeared far more upbeat, and defiant, than had been the case following the Southampton defeat when he apologised to United supporters.

"It is always like that when you lose a game, certainly in the last minute, that you are fed up," he said.

"How we have played is not always the main issue because you can play badly and win. When that happens, you are not so fed up.

"But we have played very well and lost, like Chelsea away last season, so when that happens you are more fed up.

"It is your duty as a manager and a player to stand up again and do what you do because that is a professional attitude," he insisted.

Derby, managed by former Real Madrid assistant coach Paul Clement, go into the game on the back of a heavy 4-1 loss to Championship promotion rivals Burnley, which left them fifth in England's second division.

But Clement is relishing the chance to lead his players out against United, who Derby last overcame in the FA Cup in 1897.

Derby, managed by former Real Madrid assistant coach Paul Clement go into the game on the back of a heavy 4-1 loss to Championship promotion rivals Burnley, which left them fifth in England's second division.

What a challenge, to play one of arguably the biggest clubs in world football," Clement told the Derby website.

"It's going to be nice to put ourselves up against a very good side with world-class players, and a fantastic coach in van Gaal."