The 37-year-old played the full 90 minutes and scored a goal in Poland's 2-0 World Cup qualifying win over Lithuania on Sunday.
"Lewandowski has a muscle injury in the biceps femoris of the left thigh. His recovery time will depend on how the injury evolves," Barcelona said in a statement.
Spanish media reported that Lewandowski is expected to be out for four to six weeks, making him a doubt for the Clasico at the Santiago Bernabeu on October 26th.
Lewandowski joined a growing list of absentees for coach Hansi Flick, with Raphinha, Gavi and Dani Olmo also sidelined, while goalkeeper Marc-Andre ter Stegen is still recovering from back surgery.
Barca could, however, be boosted by the returns of Lamine Yamal and Fermin Lopez, who are expected to be available after the international break.