MUNICH, Germany (AFP) - Bayern Munich's German international midfielder Bastian Schweinsteiger is out of Sunday's match against Bayer Leverkusen as he struggles to overcome a mystery injury, his club said Friday.

"We still don't know exactly what the problem is with his knee," said coach Ottmar Hitzfeld.

"He could be out another three days or it could last two to three weeks."

Schweinsteiger had been out since last weekend, suffering from bruising to his knee but now club doctors think he may have been the victim of a tick bite. Ticks are a parasite that carry viruses and are common in the south of Germany.

"That's just a hypothesis: he's had a blood test and we're waiting for the results," said Hitzfeld.

Schweinsteiger missed Bayern's Champions League quarter-final second leg against AC Milan, in which the Germans were beaten 2-0 - 4-2 on aggregate -to end their involvement in Europe this season.

It's another blow for Bayern, currently trailing down in fourth in their domestic league, as they have already lost French full-back Willy Sagnol for the rest of the season.