Former Spice Girl Mel was unable to appear on Saturday night's live show because of illness, so former judge Tulisa was drafted in as an eleventh hour replacement.
And, according to Simon, she was the only person he wanted...
Simon told The Metro's Guilty Pleasures: "She was the only name who came up. She loves doing the show, she's always in touch. She was quite quick on the phone to say she was available."
Tulisa stood in for Mel on Saturday night, supporting Mel's finalist Andrea Faustini, who finished in third place. The following night, Mel was well enough to attend the grand finale.
In a twist of fate, Tulisa's return to X Factor came just a week after she admitted she'd love to return, particularly now as she's in a better place in her life.
She served on the judging panel for two years before leaving in 2013 to concentrate on other work, including her music. As a judge, she mentored Little Mix to success. They are the first and only group to win the show.
Tulisa told Mail Online: "Going back to X Factor is something I feel I could definitely do now because of the place I'm in.
"I understand with that show that for anyone doing it, it takes a lot of energy and you have to feel it and want to do it, and you have to have that passion. The place I'm in now, I definitely have that back again, and it is something that I would and could do."
Tulisa did appear twice during X Factor earlier this series, firstly to help Louis Walsh pick his finalists at Judges' Houses in Bermuda (lucky thing!) and again to offer advice in the recording studio.
X Factor 2014 was won by Ben Haenow, who beat bookies' favourite Fleur East. His winner's song, 'Something I Need', a cover of the OneRepublic track, is top of the iTunes chart.
Reveal is now on Google Plus! Join us here for the latest pictures, fashion and celebrity news!