Yankov and Popov out of Bulgaria qualifiers
Yankov, capped 50 times for Bulgaria, tore an Achilles tendon during his Russian club Rostov's 2-1 league defeat against Kuban Krasnodar and will be out of action for up to six months.
CSKA defender Popov needs surgery on a cruciate ligament tear in his left knee after being injured in his team's 1-0 win over Chernomorets Burgas in the Bulgarian league and will also be sidelined for up to six months.
"It's a tragedy," Popov told local media. "I'm very, very disappointed but I have to accept it and I hope to be back in the spring."
Bulgaria host England on September 2 in their next Group G qualifier. Lothar Matthaus's team visit Switzerland four days later and play Wales at home on October 11.
England top the group, ahead of Montenegro on goal difference, with 11 points from five games. Switzerland and Bulgaria have five points while Wales are bottom with no points.