The musician, who joined forces with the heavy metal band to work on songs he'd written for German theatrical production Lulu, said that the material they created together could be "the best thing done by anyone, ever".
"The version of the Lulu music I did with Metallica is awe-inspiring," Metal Hammer quotes Reed as saying. "It could create another planetary system. I'm not joking, and I'm not being egotistical."
When asked if they would work on the stage show together, Reed joked: "I don't think Metallica wants to be a band on Broadway! They're metalheads. They're not going to do that."
Reed recently referred to their musical union as "a marriage made in heaven", adding: "I knew it from the first day we played together, 'Oh, man, this is perfection, right in front of me'."
He has revealed that the collaboration music will "probably" be released by November this year.
Watch the music video for Metallica's 'The Day That Never Comes' below:
Watch Lou Reed perform 'Perfect Day' live in 1998 below: