Lopez, a pop star and actress, and rocker Steven Tyler will join Jackson on the judges' panel when the singing competition's 10th season begins Wednesday night.
The New York Post said talk-show host Jimmy Kimmel asked Jackson on "Jimmy Kimmel Live" Tuesday night what he thinks of Lopez's singing.
"She would definitely get a strong, solid 7," Jackson replied. "A seven's really good. She doesn't have the biggest voice in the world, but she can sing. It's the package -- look at Britney, Ke$ha, Gaga, they're not like my girl Mariah Carey."
The Post said Jackson later elevated Lady Gaga's score to an 8.