The statement says Fennell was rushed to hospital on Oct. 15 with severe back pain and an emergency MRI revealed she had two herniated discs.
The statement says Fennell's surgeon recommended urgent surgery to avoid nerve damage complications, with the procedure slated to take place as soon as possible.
The development means Fennell won't be present at a special council meeting next week which was set to address an arbitrator's report on expenses by city council.
Fennell recently lost her bid for re-election amid a spending scandal that rocked the city northwest of Toronto.
A recent audit found she broke municipal expense rules — including on business-class flights, premium hotel rooms and cellphone IQ quizzes.
The audit found the expenses added up to more than $172,000, although roughly $41,000 was repaid and an arbitrator then found Fennell must only pay back $3,523.
Fennell’s term as mayor ends Nov. 30.