Houllier told talkSPORT: "They lost some key players. Suppose Steven Gerrard, maybe [Luis] Suarez and another defender were out against Tottenham - it could have been a different game.
"At this moment, the big loss for them was Theo Walcott. Theo was giving them some depth and some speed up front and once they lost him the results suffered.
"Particularly in big games he's a threat and he can make goals and score goals. He had improved a lot so it's a shame he's not playing at the moment.
"They lost Theo, [Jack] Wilshere and [Aaron] Ramsey, who had a terrific start to the season. It is too many key players [lost] plus the fact they look a bit more tired than the others."