The songwriter announced last week that he would leave the show's judging panel at the end of the second series to concentrate on his job as chairman of Epic Records.
But head judge Cowell does not think that Reid has come to terms with his departure, reported E! online.
"LA is regretting his decision, trust me," Cowell said.
However, Reid, 56, who is joined on the panel by Cowell, Britney Spears and Demi Lovato, knew he would only make a short-term commitment to the series, but will continue to work with the 'X Factor' boss on other projects.
"I came into this knowing that it wasn't gonna become my life. Simon and I are really good friends, and he knew at some point I needed to get back to work," Reid said.
