PSG travels to the BayArena on Tuesday for the first leg of the last 16 tie, with Laurent Blanc's side having only lost once during the group stages of this season's competition and on a five-game unbeaten run.
Houllier believes the French champion has made great strides in recent months and expects the club to prevail and book a quarterfinal place.
"It's going to be very, very difficult (for Leverkusen) because Paris have become more mature and stable in recent months," he told the official Bundesliga website. "This is a team that can go a long way.
"Bayer have a good team with a great coach (Sami Hyypia) who I had as a player at Liverpool, but Paris are simply stronger."
Houllier also believes that reigning champion Bayern Munich will take some beating in Europe's premier club competition.
"Bayern certainly have the potential, but it's a long way to get there," he added. "I really like this latest Bayern team, Pep Guardiola's doing a great job."