It is not known if the UFC will seek a different opponent for Diaz on the Nov. 3 card at New York's Madison Square Garden.
The fight with Poirier was to have been Diaz's first since August 2016, when he lost a majority decision to Conor McGregor. Diaz (19-11) is considered one of the top stars in the sport.
Poirier (24-5, 1 NC) has won his past three fights. He most recently defeated Eddie Alvarez by second-round TKO on Saturday at UFC Fight Night in Calgary.