The panellists, who are both new to the show, took to Twitter yesterday (June 8) to joke about Louis's relative anonymity in the US.
"Yeah, substitute teacher!!" tweeted Lovato, after the official X Factor USA account announced that Walsh would be taking Cowell's place due to illness.
Spears pretended that she thought Walsh was a reality star, saying: "Louis? That guy is on the real housewives or something... right? (sic)"
Spears and Lovato eventually made amends with Walsh, with Spears tweeting: "Just kidding! Love Louis. Met him at @TheXFactor in the UK! Welcome to the @TheXFactorUSA gang."
She then posted a picture of herself and the British X Factor judges (including Walsh) taken after her performance on the show in 2008.
However, the popstars later made it clear that their allegiance lies with Cowell.
"I can't believe I'm actually saying this... but I miss @SimonCowell," posted Spears. "Hope you feel better soon!"
