Walsh will join Britney Spears, LA Reid and Demi Lovato at the judges' table in Kansas City later today (June 8) as Cowell recovers from a bout of bronchitis, The Sun reports.
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"Simon said it was an emergency and Louis was the man for the job," an insider explained. "He said he'd sorted out a private jet for him and he was to get on it as soon as he finished up in Manchester [yesterday].
"He greased the wheels with the US bosses, telling them that Louis was the man who knew more about The X Factor than any other judge."
Walsh has been a judge on The X Factor in the UK since it launched in 2004, bar a very brief period during the fourth series where he was replaced by Brian Friedman.
He also previously filled in at Britain's Got Talent two years ago when Cowell fell ill with flu.
Photo gallery - 'The X Factor' USA judges at season two auditions:
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