And friends close to the England star believe that was the reason for his shocking showings against the likes of Germany.
He was meant to be one of the biggest stars on the world stage but instead was one of the biggest flops - failing to score in four games as England crashed out.
Rooney had scored goals galore for United and England up to mid-March but failed to net one goal from his last 10 games of the season, including the World Cup.
SunSport can reveal Old Trafford insiders knew of Rooney's off-field activities BEFORE the start of the finals on June 11.
A Manchester United source said: "The relationship with this woman was hardly a secret within the club - but Wayne was always hopeful it would not be known by everybody.
"Unfortunately for him, everything has been made public."
Roo looked a forlorn figure throughout the Three Lions' disastrous time in South Africa, with his poor displays a mystery to both fans and pundits alike.
His form had been such a disappointment that boss Fabio Capello made Rooney aware that he could be axed from the England team.
Rooney decided not to return home from England duty yesterday despite the revelations that he cheated on his pregnant wife Coleen with a £1,200-a-night prostitute.
Rooney's sordid relationship with Thompson lasted from June last year until October.
And friends believe the star striker, who earns more than £150,000 a week, became increasingly fearful that his cover would be blown in the run-up to the World Cup finals.
Capello is monitoring Rooney's behaviour and mood ahead of tomorrow's match in Switzerland.
Rooney will make a final decision early this morning whether to travel with the rest of the squad for the Euro 2012 qualifier.
Before the full storm broke on Saturday, the United ace had enjoyed a revival in form for club and country.
He delivered an improved display in the 3-0 win over West Ham late last month, which saw him break his goals duck with a penalty.
And he had a hand in all four goals in Friday's 4-0 Euro qualifying win over Bulgaria at Wembley.