"It became toxic for a while" – Sir Alex Ferguson names 'fantastic' Arsenal signing that made Manchester United rivalry intense

Legendary manager Sir Alex Ferguson has revealed that the signing of Thierry Henry in 1999 took the rivalry between Manchester United and Arsenal to the next level.

The 79-year-old opened up about his rivalry with former Arsenal manager Arsene Wenger in a segment of the Frenchman's biopic Arsene Wenger: Invincible.

The Scotsman explained how the signing of Thierry Henry and his transition from a winger to a striker made the rivalry even more intense. He said:

"It became toxic for a while. The big change that made Arsenal was reinventing [Thierry] Henry. And the goals he scored, he was fantastic. You're always looking in the rear-view mirror to see who's coming up behind you, and when you see someone you accelerate. Arsenal were catching us up, and their team was good enough to go above us, there's no question about that."

Thierry Henry was brought to Arsenal from Juventus in 1999 for a then club-record fee of £11 million. He joined the Gunners as a left winger, but Arsene Wenger decided that his compatriot was more suited to a central role.

The decision paid off massively for Arsenal as Henry would go on to become the club's top scorer, with 228 goals in 377 appearances across all competitions.

While at Arsenal, Thierry Henry was named Premier League Player of the Season twice. He also won the Premier League Golden Boot four times and the European Golden Shoe twice.

Henry was a major part of Arsenal's two Premier League-winning teams, including the Invincibles in the 2003-04 season.

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"The achievement stands above everything else" - Manchester United legend Ferguson on Arsenal's unbeaten season

Arsene Wenger and Sir Alex Ferguson had many battles as Arsenal and Manchester United managers

Sir Alex Ferguson was also full of praise for Arsene Wenger's achievements as Arsenal manager.

The former Manchester United boss is the most decorated manager in Premier League history, with 13 league titles to his name. However, he considers Wenger's achievement during the Invincible season to be above all his triumphs. He explained:

"I won 13 leagues but I was never near going through a season undefeated. The achievement stands above everything else, and it was Arsene's."

Arsenal went the entire 2003-04 season unbeaten, winning 26 games and drawing the other 12. They won the Premier League title by an 11-point margin.