The pair was due to take to the stage at the Children In Need Rocks event in Manchester, England, on Thursday, marking Cheryl's first performance since being dropped from the US X Factor in May. However, it could all be off if she doesn't get better in time.
Gary told The Mirror, 'She's ill. We're waiting to hear.
'It's meant to be a surprise but that's the plan - if not, I don't know what we're going to do. I might just have to get on the piano and do a bit of Roll Out The Barrel.'
But despite the drama, the X Factor judge and Take That star said he can’t wait to get back on the stage.
'I'm looking forward to it I really am,' he said.
'I'm always trying to remind myself that's actually what I do and I don't just sit behind a desk talking. So it's important to me to get back out there.'