Published Friday, 08 June 2012
Walsh, who has been a regular on the British version of the TV talent show for years, joins Antonio 'L.A.' Reid and Cowell's new signings Britney Spears and Demi Lovato to judge wannabe singers at auditions that could not be rescheduled.
Spears and Lovato poked fun at the stand-in after he was announced as Cowell's replacement on the programme.
Spears pretended not to know who he was, tweeting: "Louis? That guy is on the real housewives (reality TV show) or something....right?" And Lovato referred to the new addition as a "substitute teacher!!"
Toxic hitmaker Spears later added: "Just kidding! Love Louis. Met him at @TheXFactor in the UK! Welcome to the @TheXFactorUSA gang."
And Walsh has another non-US X Factor celebrity wishing him luck - his old UK X Factor co-judge Sharon Osbourne.
She tweeted: "Good luck @Walshlife. Remember you are the best judge in the world. Show America what a good judge is on X-Factor US!"