Officials at the Ultimate Fighting Championship were not immediately available for comment.
McGregor, a featherweight champion who is unbeaten in seven UFC fights, was looking to make history against Dos Anjos by becoming the first fighter to hold titles in multiple weight classes simultaneously.
It marks the second time in seven months that a defending champion has pulled out of a title fight against the outspoken Irishman.
Last July, then-featherweight champion Jose Aldo withdrew from a defence against McGregor with a rib injury on less than two weeks notice.
UFC 196 is scheduled for March 5 at MGM Grand Garden Arena where a female bantamweight title fight between Holly Holm, who last November handed unbeaten Ronda Rousey her first loss, and Miesha Tate is slotted as the co-main event.
(Reporting by Frank Pingue in Toronto. Editing by Steve Keating.)
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