The Brazilian is yet to return to pre-season training, but has taken to social media to claim that he has been the victim of identity theft.
Indeed, the letter, which has FIFA logos on it, appears to show Willian's signature authorising a change of agent to represent him. However, the 29-year-old has insisted that he never signed the document.
"In the recent days I have been informed that a 'letter of authorisation for player agent FIFA', allegedly signed in my name has been circulated through the football market," read an Instagram post from Willian.
"In such letter, Mr. Denirobson Dias would grant an authorization to Mr. Renee Pinheiro Anunciacao to represent me until 15 August 2018, throughout football clubs in the United Kingdom and France. I would like to clarify that I never signed such letter, the content of which I completely deny.
"Finally, since there seem to be clear evidence that my signature has been forged, or anyway misused, I hereby inform that my lawyers are already instructed to proceed with all appropriate legal measures before any competent courts or authorities. I hope I have clarified the matter and any possible misunderstanding."
Willian has two years left to run on his contract with Chelsea.