Phillip Schofield was joined by Rochelle Humes to host the ITV morning show on Monday 9 May as Willoughby prepares to host the live reboot of celebrity sports contest The Games every evening this week.
Willoughby, 41, revealed on social media: "Morning… going to miss my @thismorning family this week however I’ve had a change of career and become a Cheerleader… it was only a matter of time @thegamestv."
The Dancing On Ice and Freeze the Fear presenter will be joined by ex-cricketer and Top Gear star Freddie Flintoff to host The Games.
The sporting reality TV contest was originally broadcast on Channel 4, where it ran for four series between 2003 and 2006.
It relaunches at 9pm on ITV every evening from Monday, 9 May to Friday 13 May featuring 12 celebrities who include Strictly Come Dancing pro Kevin Clifton, ex-Coronation Street star Ryan Thomas, and model and TV star Christine McGuinness.
Professional dancer Clifton told Good Morning Britain that his dance background was not helping his Olympic efforts as much as viewers might expect.
He revealed: “People keep saying for each event, ‘Oh, you’re a dancer, you’re going to be good at this’, but for most of them, doing the 400 metres or doing the hurdles or something, I don’t know how much doing the paso doble translates to it.
Clifton added: “I keep doing this thing that we call spotting in dancing where you keep turning and leaving your head in one place.
“So rather than swinging around with a hammer, I’m sort of giving you a flamboyant dance turn and I’m trying to get out of that habit.”
Other contestants - who have been training for weeks ahead of the show - are former Love Island stars Olivia Attwood and Wes Nelson, Mel B's daughter Phoenix Brown, The Wanted's Max George and Harry Potter actor Josh Herdman.
Coronation Street's Colson Smith and Emmerdale's Rebecca Sarker are also taking part, along with newsreader Lucrezia Millarini and footballer and singer Chelcee Grimes.
The contestants have had a crash course in various Olympic disciplines which will cover the athletics arena, gym, swimming pool and cycling track.
A big-name line-up of sporting stars will make guest appearances each night to offer advice and insight, beginning with World Heavyweight boxing champion Tyson Fury and Olympic Gold medallists heptathlete Denise Lewis and diver Matty Lee.
Watch: Holly Willoughby giggles as she discusses health benefits of self-pleasure