The striker was on the receiving end of a robust challenge from Scotland's Gordon Greer early on in the sides' 2-2 draw at the Stadion Narodowy - a tackle Lewandowski described as "brutal".
With Bayern returning to Bundesliga action on Saturday against Werder Bremen, the former Borussia Dortmund man explained he was to undergo tests on his knee.
Bayern, whose injury problems worsened with the news that Thiago Alcantara suffered another knee ligament injury in midweek, are now set to wait on the severity of Lewandowski's injury.
"It was a brutal foul and I was in pain until the end of the game," said the Pole, who left the stadium with heavy dressing on the wound.
"I hope the injury is not serious, but I will get tests.
"With every step, I had to clench my teeth and hope my team-mates would help me out, which they did.
"The tackle didn't just rip my sock, it broke my shinpad. You can hardly break a shinpad with a hammer, so that shows how hard the tackle was.
"I dread to think what would have happened if I didn't have the shinpad on."
Bayern are also without Bastian Schweinsteiger (knee), Pepe Reina (calf) and Holger Badstuber (thigh) for Saturday's fixture at the Allianz Arena.
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