Houllier said: "He can play wide right, on the left or as a striker. He is strong, he is solid and he can beat players. He is also a good guy with a good personality, which is why the criticism he has received has been very unfair.
"He went to the World Cup, played throughout the competition and reached the quarter-final. He then came to a new country when he was only 21 and started well.
"But, like most of the World Cup players, he had a dip. Virtually every player who went to the World Cup has suffered injuries or a dip in form with the possible exception of Paul Scholes, who has been playing very well.
"The criticism of him is very unfair because he also had to settle into the English game. Robert Pires, Sylvain Wiltord, Dennis Bergkamp - it took all of these players time to adjust to the English game and El-Hadji is no different.
"The game here is more physical. To be a success here you need to overcome the physical side of the game and you need to have character.
"In this country you have to be very strong mentally as well as physically. I insist that a player needs time to learn how to play in English football, even if he has the skill and the will.
"The pace is also higher than anywhere else, which is why it takes time for continental players to adapt."