TV presenter Dermot, 41, stunned fans on Friday (27 March) when he announced he was leaving the ITV talent show after eight years.
And now Simon, 55, has broken his silence on the news.
Dropping the Mr Nasty routine, Simon sent Dermot a sincere message via Twitter describing him as "an amazing presenter".
Referring to Zayn Malik's bombshell decision to leave One Direction this week, Simon began by joking: "A quiet week at the office this week!"
He went on to say: "Dermot has been an amazing presenter on the X Factor.. He held the craziness together. Most of the time. He is a great guy and a real talent.
"And we will work together on something else."
Dermot has been an amazing presenter on the X Factor.. He held the craziness together. Most of the time. He is a great guy and a real talent— Simon Cowell (@SimonCowell) March 27, 2015
Dermot in turn thanked everyone for their support after announcing the big news.
He tweeted: "Well... That wasn't exactly an ordinary day.
"Thanks for all the love and best wishes today. Really appreciated. Best decision, me thinks. But made with love and respect to all."
On Friday afternoon, Dermot broke the news he was leaving by saying: "After eight wonderful years on @TheXFactor it's time for me to move on.
"I'd like to thank ITV, Simon, The X Factor family and particularly the viewers, all of whom have been a big part of my life for so long.
"I'd like to wish the team all the best for the future, especially whoever takes over from me.
"You're about to be the conductor on the most brilliant, runaway train in showbiz. Good luck you'll love it."
Dermot has been presenting the ITV talent show for eight years after taking over from Kate Thornton in 2007.
In January, Dermot revealed he would love Emma Willis to take over from him.
Reports now say Olly Murs and Caroline Flack are the hot favourites to take over Dermot's role.
X Factor's most shocking moments...
For exclusive competitions and amazing prizes visit Reveal's Facebook page NOW!