"I didn't know," Jackson toldThe Los Angeles Times in an interview published Thursday.
"I got an announcement from the Television Critics Association. So I called him and was, like, 'Dude, you gotta tell your boy, come on!' He said it was so crazy that day."
Cowell inked a deal with Fox earlier this month that will end his American Idol judging role while also launching his British The X Factor reality competition series for the network in Fall 2011.
Jackson has previously stated he thinks Cowell might have "so much fun" with American Idol's currently-airing ninth season that he'll decide to come back, however he also acknowledges the opposite might happen.
"You know, there's a song by the Fray that I really love. It's called 'Never Say Never,'" he told The Times when asked if there was a chance he might reunite with Cowell on The X Factor once his own Idol contract expires.
In addition, Jackson said he's "really sad" Cowell won't be back since the American Idol landscape already changed when Paula Abdul -- who Cowell has already suggested may join him on The X Factor's still-announced judging panel -- announced her departure last summer.
"This is a journey that me, [host Ryan Seacrest], Paula and Simon started together and it's changing a bit," he told The Times.
Jackson added that it's good The X Factor will be on in the fall since he doesn't think there's enough room on television for two singing-themed reality competition series "happening at the same time."
"But I think there's definitely room for two to four music competition shows if they're done well and people have an emotional involvement in these contestants," he told The Times.