England boss Andy Robinson is already without Wasps flanker Joe Worsley for the November schedule of appointments with Australia, New Zealand and Samoa.
But as Worsley begins the recovery process from knee trouble, Gloucester have revealed that 6ft 6in lock Brown has a disc problem in his neck.
Brown, widely tipped to make a Test debut against the Wallabies on November 12, has been ruled out of Saturday's European Challenge Cup clash against Bayonne.
"Alex has a disc problem and he will spend the next seven days being assessed," said Gloucester head coach Dean Ryan.
"Hopefully, after that time, we will get some positive news."
Brown added: "It is disappointing that the injury has happened at this stage, but I will work hard with the medical team over the next week and aim to resume playing as soon as possible."