"He announced he's leaving on my first day. I'm trying not to take it personally," joked DeGeneres during Monday's taping of her The Ellen DeGeneres Show, which aired Tuesday.
"Seriously, I'm going to be very, very sad to see him go, because I think he has made the show what it is. I think Simon Cowell is a huge part of that show."
The much-rumored deal between Cowell and Fox -- which will include him launching his British The X Factor reality competition series for the network in Fall 2011 -- was finally formally inked in front of reporters at the Television Critics Association's press tour on Monday.