The shock rocker hung out, separately, with Lennon and Paul McCartney after The Beatles split, as both were members of Cooper's infamous Hollywood Vampires drinking club.
And Cooper says the pair remained "best friends" despite the band's sour demise in 1970.
Asked by QFM96 if they would have reunited had Lennon survived, Cooper says: "Absolutely. Here's the thing about them. When they were after each other's throats, when it came to the breakup and all that stuff, if anybody in the Vampires back in those days – that was our drinking club – if anybody said anything bad about Paul, John would take a swing at you, because that was his best friend.
"If anybody said anything about John to Paul, Paul would walk out of the room. He'd just walk out. Because you are not allowed to talk about their best friends. They were best friends no matter what was going on in the whole thing.
"One went one way and one went the other. I think John wanted to be more political. Paul was not into that that much."
Recently, McCartney revealed that he sourced the technology to "extricate" John Lennon's voice from an old demo so that he could use it to complete an already-written piece of music.
Of the track, which is has not yet been officially revealed, but is rumoured to be the unheard 1978 John Lennon song Now And Then, McCartney says: "We just finished it up and it'll be released this year."