The Welshman formed the influential group with Reed in 1965 and often collaborated with him after he left the band in 1968. He and drummer Maureen Tucker are the only remaining original members of the Velvets.
Taking to his Facebook.com page on Sunday, Cale wrote, "The world has lost a fine songwriter and poet... I've lost my 'school-yard buddy'."
Velvet Underground guitarist Sterling Morrison died in 1995 and Nico, who contributed vocals to four songs of the band's debut album, died in 1988.
The Doors star John Densmore has also offered a tribute to Reed via Twitter.com, writing, "The soul of Nyc has broken on thru (sic). Much respect, Lou."