The Brazil playmaker wrote a poem in his native Portuguese in tribute to his mother on Instagram on Thursday after her death, reportedly of cancer.
Chelsea sent their condolences to the 28-year-old and his family.
Chelsea said on their website: "All of us at Chelsea Football Club are sending our love and deepest condolences to Willian and his family after the passing of his mother
We are thinking of you Willian."
Willian scored in Brazil's 2-0 win in Venezuela on Tuesday night.
The Blues have won one of their last four Premier League matches - a 2-0 win at Hull prior to the international break - but beat Leicester in the EFL Cup last month.
Source : PA