Byrne was eliminated from The X Factor on Sunday night after Cowell joined Dannii Minogue and Cheryl Cole in voting to save Cher Lloyd in the controversial sing-off.
Asked if Byrne could have won Britain's Got Talent, her mentor Walsh told LBC 97.3: "I think she's a winner - Simon is arranging a record deal for her with Sony.
"Susan Boyle didn't win and she sold so many million records. I think Mary can be up there with the kind of [Britain's Got Talent series one winner] Paul Potts, Susan Boyle thing, which Cowell does very, very well."
Bryne recently claimed that she has a "a big career" in front of her despite her exit from the reality talent contest.