Houllier was admitted to hospital on Wednesday night after falling ill. He will miss tomorrow's game and could now be out for the rest of the season.
"Obviously, everyone at the club is concerned for the manager," Downing told the club's official website. "It's never nice, an experience like this, especially for his family. But the news is that he's going to be all right and that's pleasing and reassuring.
"A good win against Stoke on Saturday will help him no doubt. In fact, the last couple of weeks have been good and I'm sure the gaffer will want us to continue in this vein."
Assistant manager Gary McAllister takes charge for the match at Villa Park. Kickoff is at 15.00.