Houllier has shaken hands with Villa owner Randy Lerner on a three-year contract with the Midlands club but he will not officially be released from his French Football Federation contract in time to take his place in the dugout at Stoke.
Houllier was still due to hold a press conference on Friday afternoon, but Kevin MacDonald will be in charge for a sixth successive game against Tony Pulis' side, with Houllier expected to be in the dugout for the first time for the home clash with Bolton on September 18.
He will take complete control after the weekend and will not formally meet the Villa players until next week as they are on a day off on Friday.
Houllier plans to hold talks with MacDonald at Villa's Bodymoor Heath training complex to discuss his role in the new set-up.
Former Liverpool midfielder MacDonald is currently reserve team coach.
Houllier revealed he rejected other offers before taking charge at Villa and hopes to build on the success achieved under O'Neill.
He said: "It was really starting to tickle me. I wanted to go back pitchside.
"I had, of course, other proposals but this one seemed to be the most interesting in terms of working conditions and environment.
"Aston Villa is not a club on the same level as Liverpool, let's be objective and honest.
"It's a club that according to me has belonged between seventh and 12th place in the Premier League.
"But, above all, it's a historic club which has won the European Cup.
"It's a club which has got good infrastructure and a fine team which is after one thing - to improve year on year."