Spurs were hammered 6-0 at Manchester City last weekend and whilst they have since bounced back with a Europa League win in Norway, the heat will really be turned on manager Andre Villas-Boas should his expensively assembled outfit fail again.
Suffering heavy losses was not something Irwin experienced that often during his 12 years at Old Trafford.
It did happen though. And it was something the former Republic of Ireland defender has not forgotten.
Speaking at Chevrolet retailer Frosts of Shoreham, Irwin said: "You would expect a reaction.
"It is not very nice when you get beaten by a scoreline like that.
"You put an extra bit of work in on the training ground. Everyone is more focussed.
"In front of their home supporters, Tottenham will be very motivated so Manchester United will have to be on their guard."
It makes a change for David Moyes to be facing a manager who is under more pressure than him.
After all, since the defeat at Liverpool at the end of August, United have struggled for consistency.
Even their present 11-match unbeaten run has featured results the Red Devils would class as disappointing, not least at Cardiff last Sunday when they conceded an injury-time equaliser that kept them out of the Champions League places and only a point ahead of Spurs.
"When you are playing against Chelsea, Liverpool and City in your first five games, you know it is not easy, either on paper or the field," said Irwin.
"Losing to West Brom after that was not very good.
"Since then United have come back into a bit of form but the international break came at the wrong time for them and they didn't play well against Cardiff.
"At the moment it is two steps forward, half a step back."
Wednesday's five-goal hammering over Bayer Leverkusen was a significant advancement.
Yet Irwin doubts United can start taking their form for granted.
Not that he believes it is any different for his former club's major rivals, none of whom make a cast iron case to be crowned Premier League champions next May.
"There are four or five teams who could win it," said Irwin.
"That is the beauty of this season's Premier League.
"It is not going to take 90 plus points to win it this year that's for sure. It will be mid-80s at best.
"It is going to be really entertaining."
And, with a list of key fixtures looming in the busiest month of the entire campaign, Irwin doesn't expect anything significant to change until after the turn of the year at the earliest.
"It is going to be one of those topsy turvy seasons," he said.
"I have no doubt that come February time, two or three will break away from the pack.
"But up until then you are going to see teams dropping a lot more points.
"Part of it is there are a lot of new managers.
"That means there are a lot of clubs where the manager is going to take time to find the right team."
Denis Irwin was presenting Frosts of Shoreham with a Manchester United Chevrolet Trax - for more information go to www.chevrolet.co.uk.