Lewandowski, who made his 100th LaLiga appearance on Saturday, went down clutching his leg late in the game, which Barca won in the 98th minute to extend their LaLiga lead.
"Tests carried out this Sunday have confirmed that first-team player Robert Lewandowski has an injury to the semitendinosus muscle in his left thigh," Barca said in a statement.
"His return to team training will depend on how the injury progresses."
The club did not give a timeline on his return but a hamstring injury may rule the 36-year-old - who has scored 40 goals this season - out of the Copa del Rey final against Real Madrid next weekend.
Spanish media reported he might be sidelined for three weeks, a potentially huge blow for Barcelona who face Inter Milan in the Champions League semi-finals in the coming weeks.
Lewandowski is the leading scorer in LaLiga this season with 25 goals, three more than Real Madrid's Kylian Mbappe.