Hammered 4-1 by David Moyes' men on the opening day of the season, Swansea turned the tables in magnificent style on Sunday, with Wilfried Bony's last-minute header securing their first ever away win over United.
Even those of a red persuasion could not begrudge Swansea their success, which most felt was richly deserved, even without Fabio's dismissal for a reckless challenge on Jose Canas providing them with an additional advantage.
But De Guzman came the additional lift came from those same disgruntled supporters and the disenchantment felt at their own team.
"You could tell their fans weren't pleased when they played the ball backwards and we used that to our advantage," said the 26-year-old.
"You could feel it (edginess in the stadium). The fans wanted to win.
"They are at home. Their crowd is an extra man. But if they are turning their backs or things are not going well and the team is not playing the way they should, as an opposition player you have to use that.
"You could hear the fans booing at the end of the game and when they played the ball backwards we knew that as well."
For all Swansea's industry, the United team they faced at Old Trafford was light years away from the one that travelled to the Liberty Stadium in August, when it appeared the transition from one Scot to another would be very short indeed.
"I didn't play in that game but I do remember they were lethal, more so than they are being now.
"But don't forget, they still have two great players missing in Robin van Persie and Wayne Rooney."
Nevertheless, the midfielder confirmed a win at Old Trafford represented one of the greatest days in his career.
And, after lifting the League Cup last season, the thirst for more honours is high, even it may be some time before Swansea's fourth-round opponents are known given they must wait for the Crawley, Bristol Rovers and Birmingham conundrum to sort itself out.
"We will be ready for the next round," said De Guzman.
"If we can beat Manchester United, we can beat anyone.
"We have had a taste of Wembley and we are kind of getting used to it. It would be great to get there in another competition."