Javi Martinez has revealed doctors have told him they are 99% certain he has suffered a broken anterior cruciate ligament in Bayern Munich's 2-0 DFL-Supercup defeat to Borussia Dortmund.
The Spaniard was involved in a collision with BVB defender Marcel Schmelzer shortly after Henrikh Mkhitaryan had given Jurgen Klopp's side the lead.
The former Athletic Bilbao man went down immediately clutching his knee and was unable to continue, with Dante replacing him shortly afterwards.
Initial reports claimed the midfielder's injury was a potentially serious one and Martinez admitted the early prognosis appears to have been accurate.
"I'm on the bus and just spoke with the doctor. He told me 99% I got my ACL broken. I rely on the 1% left but I think I'm in a bad way," he said.
"I'm on the bus with a cold machine. It's so hard... If it's an ACL injury, I'll have surgery tomorrow and I'll start my recovery to come back as soon as possible and be a better player. I'll be back."
Guardiola had started the match with Martinez, a midfielder by trade, in a defensive role as Pep Guaridola looked to experiment with a 3-4-3 formation.