The Brazilian arrived at Stamford Bridge from Anzhi Makhachkala in August for £30m and appeared to struggle in the early stage of his Chelsea career.
But Willian has improved greatly in recent weeks with impressive performances in the wins over Liverpool and Southampton, where he came off the bench to help the Blues to a 3-0 victory.
Willian's form has been so good that two-time player of the season Juan Mata has become a bit-part player this term, and Cech is confident there is more to come from the South American.
Cech said: "Yes. You always give players a few months to settle because the Premier League is different to anywhere else.
"Questions were raised because he came from Anzhi Makhachkala where he was unlucky with injuries and he didn't play much.
"But from day one he has worked really hard. He was waiting for his chance and since the first start you can see he is taking his opportunity.
"He has been absolutely brilliant. He keeps working for the team, making the difference, scoring goals, defending. I thought I'd never see anyone running more than Ramires, but he's really there.
"It's fantastic to see he's enjoying his football, enjoying his life in the club. This is important. It's very pleasing to see his progress."