Javi Martinez says there is a "99 percent chance" that he tore an ACL in his knee in Bayern Munich's 2-0 defeat to Borussia Dortmund in the German Super Cup on Wednesday.
Martinez was airborne while trying to volley a cross when his follow-through struck a Dortmund defender. The Spain international fell to the ground in pain and was replaced by Dante in the 31st minute.
"I've knackered my knee and right now, I'm on the team bus. I've spoken to the doctor and he's told me that there is a 99 percent chance that I have torn my cruciate and external ligaments," the player said on a video published on his Facebook page.
"If it is the cruciate ligament that is damaged, they will operate tomorrow and I'll begin a recovery programme so that I can return even stronger. I'm determined to come back a better player, I'll be back."
The defensive midfielder is set to undergo scans on Thursday.
Bayern coach Pep Guardiola expressed concern, saying "It will be a long absence" before Martinez can return to the pitch.
"We'll know more tomorrow. But I think he'll be out for a long time," Guardiola said.
Bayern sporting director Matthias Sammer agreed, saying the injury "looks bad. It's a big problem."