The 63-year-old Frenchman, who underwent heart surgery in 2001, has been absent from his job since he was hospitalized in April with a heart problem.
Villa said in a statement on its website on Wednesday that doctors had advised that Houllier "will require further time to recuperate." Houllier took over in September 2010 after the abrupt departure of Martin O'Neill shortly before the start of the season.
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