The Sun reports that the singer was chosen after impressing Simon Cowell at the final round of auditions in Manchester last week, where she was the latest in a string of guest judges standing in for new mum Dannii Minogue.
She will help choose the final 24 from the 100 remaining hopefuls at boot camp, due to take place at London’s Wembley Arena.
“There was no way Cheryl was going to be able to do Boot Camp, despite being on the mend,” an insider told the paper.
“Nicole was great in Manchester and Simon really likes her. She knows Wembley will be a tougher gig, but she’s proved she’s feisty and entertaining.”
Cheryl, who left hospital earlier this week, is expected to be well enough to rejoin the show for the judges’ houses stage in August.
The 27-year-old – who was diagnosed with the potentially fatal disease after collapsing a fortnight ago – is now recovering at home and is reportedly “on the mend”.
“She’s weak and has lost weight, and she’s still going hot and cold,” the source added. “But she isn’t bed-bound any more. She’s been watching daytime TV on the sofa.”