Schalke boss Andre Breitenreiter admits the club have been dealt a major blow following the news that captain Benedikt Howedes faces three months on the sidelines.
The 27-year-old picked up a thigh injury during last week's 2-0 friendly win over Fort Lauderdale Strikers and immediately returned to Germany from Schalke's mid-season training camp.
Further tests have indicated tendon damage, and Schalke sporting director Horst Heldt confirmed Howedes could be sidelined for the next 12 weeks.
Breitenreiter told Schalke's official website: "That's very unfortunate for us because Bene is a very important player, and we'll have to do without him for a number of weeks.
"It will not be that easy to compensate the loss, he's a leader and our captain after all."
Heldt also confirmed Schalke may be forced into the transfer market, with Howedes' fellow centre-back Matija Nastasic also sidelined with an Achilles injury.
"We had actually finished our transfer efforts but, given the duration of the absences, we now have to think again," he said, as quoted by Kicker.
The news will also come as a concern for Germany boss Joachim Low ahead of the Euro 2016 finals.
Low has already ruled the versatile Kevin Grosskreutz out of contention for the tournament in France as he failed to play a game during the first half of the season with Galatasaray.