Villa's owner Randy Lerner, who appointed Houllier in September, and chief executive Paul Faulkner have visited the Frenchman in hospital during the weekend.
There is no news on how long 63-year-old Houllier will have to remain in hospital. Villa and Houllier will be guided by the specialist over when he is deemed fit enough to return to work.
Assistant manager Gary McAllister took charge of the side for Saturday's 1-1 draw with Stoke at Villa Park. McAllister, who played under Houllier when he was in charge of Liverpool, and Villa coaches Gordon Cowans, Rafa Gonzalez and Robert Duverne have also made hospital visits.
Former Leeds and Coventry boss McAllister will remain in charge of the first team in Houllier's absence. Villa have four remaining matches in the Barclays Premier League against West Brom, Wigan, Arsenal and Liverpool.
McAllister said: "The many messages of goodwill from fans and people in and out of the game underline the esteem in which Gerard is held."
Source: PA
Source: PA