Schalke are favourites to land Chelsea midfielder Michael Ballack.
Ballack is out of the World Cup following Kevin-Prince Boateng's crude challenge on him during the cup final.
Ballack was forced out of the final after the challenge, which was only punished by a yellow card, and MRI scans showed he had torn ligaments in his right ankle.
German officials do not expect the 33-year-old to be able to train for at least two months.
Germany boss Joachim Low labelled the tackle "brutal" and said: "We were all very concerned. Michael is our captain and one of our leading players."
Ballack's contract at Chelsea runs out this summer and, although the club have offered the midfielder a new deal, it is likely he will return to the Bundesliga with Schalke.