Houllier said: "We know we are not at United's level. We know sometimes we can compete with them, but we are not at their level.
"We were challenging them all the way last season, but we couldn't keep it up all through the season this year. I think it could take two or three years to bridge that extra 20 per cent.
"This season has maybe taught me that. I still have some ingredients missing in terms of players, in terms of time and work. I am still searching.
"I don't set a time scale on when we will make that step. I guess my health is the only time scale