The stock ended the day down 2 percent.
Ackman made his remarks on "Squawk Box" after The Wall Street Journal reported that activist investor Carl Icahn had pondered selling his stake in Herbalife, worth approximately $1 billion, to a group that included Ackman.
"Carl [Icahn] is kind of the last man standing, and I think if he's gone, the whole confidence thing blows up," Ackman told CNBC.
Ackman has been short Herbalife since late 2012.
Icahn "came to me" with a proposal in early August to "cover my short" position, Ackman said.
Asked why Icahn would want to sell his stake in Herbalife, Ackman responded: "I think he knows this is toast" and "he's made bunch of money."
Icahn said: "I am not commenting on rumors and stories about what I am doing."
Entering Friday trading, Herbalife shares had been up 1.5 percent year to date, but have pulled back by approximately 9 percent this month.
— CNBC's Matthew J. Belvedere contributed to this story.