Thompson, Houllier's assistant at Liverpool and a close friend, was surprised by the announcement.
He told Sky Sports News: "I spoke to Gerard yesterday and he was getting back to full health. It comes as a bit of surprise.
"He was fine and he was looking forward to pre-season and everything.
"Things have gone on today to change matters but as far as health matters go he sounded good, he was looking forward to it."
Asked if he felt the Frenchman's managerial career was now over, Thompson added: "The game would be poorer if he packed it in. He's a fantastic manager, a great organiser and a great man manager and that's important in the modern day."