The Blues' No 1 is widely regarded as one of Europe's best between the sticks.
But a series of high- profile errors has seen him turn into one of the Premier League's dodgiest stoppers.
The Czech Republic star has struggled for confidence since suffering a fractured skull in 2006. But for Ancelotti, Cech is simply blunderful.
The Chelsea boss said: "Sometimes he comes out and makes mistakes but it's important in England for goalkeepers to come out and claim the ball in the box.
"It's not easy to find this kind of keeper. I want a keeper who will take risks. A very good keeper will have that personality and courage and this is very important. I don't want him to change anything he is doing.
"I don't think Petr is afraid. He takes risks different to other goalkeepers. Cech's injury is in the past. If he had some problems, then he wouldn't come out to catch the ball at all."
Ancelotti revealed former West Ham players Frank Lampard and Joe Cole will feature against the Hammers today.
But it is Upton Park boss Gianfranco Zola who is likely to get the biggest cheer from the Stamford Bridge fans before kick-off.
Ancelotti admits he still regrets selling Zola to Chelsea while Parma boss in 1997.
Zola went on to become a cult hero for the Blues and one of the Premier League's greatest players. His time as manager has not gone so smoothly with West Ham second from bottom.
But Ancelotti insists: "I'm a fan of Zola. I'm a fan of the good guys and he is one of them. A good coach, I think.
"The spirit you have as players can help in the relationship with players you manage.
"This is his first experience as coach and he has the quality to come through."
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