Horner died in a plane crash on Monday. He was 61.
Cameron and Horner, who first worked together on 1986's "Aliens", teamed up on iconic films "Titanic" and "Avatar".
"He was a very young 61 and raring to go on that. I'm not thinking about the problem of replacing James right now. I'm just feeling really, really sad that I won't experience that teamwork again. Both of us were really looking forward to doing it again," Cameron told the Entertainment Weekly while paying tribute to Horner.
"He was the heart of the film, absolutely. And you could say that literally, too, because he wrote the music for the song, 'My Heart Will Go On,' which was so much of the pop-culture propulsion of the film. The score album is still to this day the highest grossing instrumental soundtrack album of all time," Cameron recalled.
The director said he had met Horner just six weeks ago for a live performance of "Titanic" in Royal Albert Hall in London.