Carlos Takam will enter the ring instead in a bid to snatch the IBF belt off Joshua at the Principality Stadium in Cardiff a week on Saturday.
Pulev, who had one blemish on a 26-fight record, has been forced to pull out of the clash due to a shoulder injury that he suffered during a sparring session.
Due to IBF rules, the next challenger in line is called up and in this case it is Frenchman Takam, who at the age of 36 has fought in 39 professional bouts, winning 35, losing three and drawing one.
Matchroom Sport promoter Eddie Hearn said in a statement: "When the Pulev fight was announced I made a deal with Takam's team to begin camp and be on standby for this fight. When I called them this evening they were overjoyed and good to go.
"It's a difficult position for AJ having prepared meticulously for the style and height of Pulev, he now faces a completely different style and challenge in Takam – this hasn't happened in his career before but he is ready for all comers on October 28."
Joshua has won all 19 of his fights by way of knockout.