Willian has a two year offer from Chelsea, and the 31-year-old could stay.
He has wanted a three-year deal in previous discussions with the club.
"It is his decision," said Lampard to reporters on Willian.
"I have not got that answer. I know the situation from the club's end. I have a great relationship with Willian. I actually do not know what that decision is.
"And if that happens over the next whatever days as he said then that will be good for me either way. Of course I am very happy with Willian, as he has been brilliant for me this season with his input and work ethic within the squad.
"But it is his choice and I respect his choice. He has been a great servant for Chelsea if he decides to move on. And if he does decide to move on then Chelsea moves on as well and we have to look forward ourselves."
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