The Villa boss, 63, who had a heart bypass operation when he was in charge at Liverpool 10 years ago, was taken to hospital on Wednesday after complaining of chest plans.
His No 2 Gary McAllister took over managerial duties for Villa's 1-1 draw at home to Stoke on Saturday and will be in charge for the remainder of the campaign.
Game over: Houllier will not return to take charge of Villa this season
Villa board member Charles Krulak said: 'As you might know, he has suffered a problem that he has suffered before . . . at Liverpool. It is serious, something that we need to be concerned about.
'(Villa owner) Randy (Lerner) flew over to see him and everyone at the club has been extending best wishes. I am afraid that he will not be on the sidelines for the remainder of our games. The key is his health and that he recovers quickly.'
Not forgotten: Assistant manager McAllister leaves Houllier's dugout seat free
Aston Villa 1 Stoke 1: Bent sends get well message to Houllier Villa stand by Reo-Coker over O'Neil challenge as Brady confirms West Ham man's career in doubtASTON VILLA FC