Bulgaria suffer more injury misfortune
Yankov broke his hand after falling from a bicycle last month. He joined up with Bulgaria's training camp after being called-up by coach Stanimir Stoilov but withdrew on Tuesday after medical staff said his involvement in the matches on October 10 and 14 could aggravate the injury.
Stoilov has already lost defender Alexander Tunchev and midfielder Georgi Surmov from his initial squad.
Yankov will be replaced by CSKA Sofia's Kosta Yanev.
Bulgaria, third in Group Eight - five points behind second-placed Ireland, must win their last two games to have a chance of making the playoffs for a place in South Africa.