"This is one of the most important works of art to be offered at auction in decades," he said.
Christie's called it "the very embodiment of Blue Period aesthetic, rendered in bold, loose, swirling brush strokes that recall El Greco and trumpet Picasso's virtuosity".
Picasso met Angel, the subject, in 1899 and they twice shared studios in Barcelona.
According to the auctioneer, artist De Soto "was more dedicated to drinking and partying than to art", and the passtime forced Picasso to move to another studio in order to work. De Soto died in 1938 during the Spanish civil war.
The painting was acquired by the Andrew Lloyd Webber Foundation at auction in New York in 1995 for $29.2m.
Since then, three works by Picasso have been sold for over $50m and, in 2004, "Garcon a la pipe" became the first painting to sell at auction for over $100m.