Diao was brought to England by Houllier when in charge of Liverpool.
He said: "Gerard did a great job at Liverpool. He is a no-nonsense manager and any team of his will be well organised and very hard to beat.
"He's a hard-working guy who likes his teams to work hard and keep their shape.
"Villa have good players, so if they stick together and are ready to work hard, I don't see a problem for them this season.
"When he left, people were saying they wanted Liverpool to win the title, but I don't think when he was there that he had the money he needed."