Goretzka told German publication 'Kicker' that he is close to making a decision over his future with a number of European clubs interested in signing the 22-year-old on a free transfer.
He said: "I have to find out for myself what the next step should be.
"I have to consider a million things. I am a person who is very meticulous in such decisions. I play through all the scenarios. So I need time.
"There doesn't exist a deadline, but I think we are in the final phase.
"'The final decision will be made in January."
Arsenal and Manchester United are among the host of clubs tracking the Schalke midfielder, but according to the 'Daily Mail', Barcelona look the most likely to sign him in the summer.
With Goretzka's contract running out at the end of the season, he would be free to negotiate a pre-contract agreement with foreign clubs from January 1, and Barcelona general manager Pep Segura has reportedly made contact with the player in the last few weeks.
The Catalan giants believe the prospect of playing at the Camp Nou, as well as a sizeable signing-on fee, would be enough to convince the German international to chose Spain as his destination when he makes his decision next month.