Cologne hope Neven Subotic will sign a permanent contract once his loan from Borussia Dortmund expires at the end of the season, but fear he could be tempted to move to the Premier League.
Subotic, 28, joined Cologne on a six-month deal in January after enduring a difficult 2016 in which injuries and illness limited his playing time and saw a proposed move to Middlesbrough fall through.
The Serb made his Cologne debut a fortnight ago, and sporting director Jorg Schmadtke is already looking ahead to the summer.
"The question is what conclusions Neven will draw from a hopefully successful Ruckrunde [second half of the season]," Schmadtke told Bild.
"Will he say that he's returned to his old form and that all doors are open for him?
"Or will he say that he returned to being his old self, that the time in Cologne did him good and that he would like to stay?"
But Schmadtke stressed no decision would be made before the end of a season in which Cologne could qualify for a European competition for the first time since 1992-1993.
"It's not an issue before May," he added.
Dortmund have signed defender Omer Toprak from Bayer Leverkusen for next season and have yet to decide whether Subotic, under contract until 2018, will be part of their plans for the new season.