The Brazilian has been linked with Chelsea and Tottenham and is reportedly keen to leave the club.
Coach Mircea Lucescu said: "It is not very nice to say that Willian did not return from holiday on time and thus he is exerting a certain pressure on the club. I think it was unprofessional of him.
"The president decided to do everything possible to sign a player of Willian’s level, who already has experience of playing at a high standard. We look forward to having Willian again and I would like him to come back and behave as he did before – as a professional and decent man.
"I would like him to solve his problems in a civilised manner here in Donetsk, and not by phone. I have great respect for Willian and I would not like him to be put under pressure by the people who surround him, and who perhaps advise him the wrong things.
"He should reject the advice of those who surround him – they are not doing very good things. With or without Willian, Shakhtar will move on."