For the first time in almost 15 years, England have had to go through a qualifying campaign without the services of either Terry or Ferdinand.
"They were two huge players and they retired at similar times so there had to be a transitional period and that's what it is at the minute," Cahill said.
"We have some quality centre-backs as well, but when two big names, two big top-class centre-halves come out of the set up there is that worry. People are always going to talk about it."
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