Williams is a top target for Barcelona, which are willing to pay the Spain winger's €58m buyout clause.
But De Marcos said, "For Nico it is a delicate situation because there is a lot of outside noise.
"He is in an ideal place in a locker room that he loves with his friends and his brother. I think he has that feeling of belonging.
"What worries me most is that he makes the decision. If he makes it on his own he will be right and he will be respected. The only thing I want is for him to make the decision freely, because he has a lot of pressure."
The midfielder also said: "He is the one who has to take the last word and we trust that he will continue with us.
"There is a tide that seems that when Barça wants you there is a logic that you have to go to Barça and that if not you are making a mistake. You have to keep in mind that not all that glitters is gold, there is to know where we are."