Aston Villa have made no official statement, but it is believed that the Frenchman is in a comfortable condition.
Houllier had open heart surgery in October 2001, while manager of Liverpool, after suffering chest pains during a match against Leeds.
He returned to full-time management with the Reds five months later, but parted company with the club in 2004.
He then had a spell with Lyon before returning to full-time management with Villa in September, after consulting his heart surgeon before accepting the job.
Gary McAllister is likely to be in charge for Saturday's home Barclays Premier League clash with Stoke.