Lewandowski sustained a back injury during Sunday's match against Real Sociedad, in which he also had a goal disallowed amidst controversy in the 1-0 defeat.
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"Lewandowski is having trouble in the lumbar region of his back, and will be rested for about 10 days," Barcelona said in a statement.
The 36-year-old will miss Poland's upcoming Nations League games against Portugal and Scotland, but should be back for Barcelona's next league outing at Celta Vigo on November 23.
Yamal missed Sunday's game in San Sebastian with a right ankle injury and is expected to be out of action for between two and three weeks, the club said.
The 17-year-old will miss Spain's matches on Thursday in Denmark and next Monday against Switzerland, and Barca's trip to Vigo.
He is also a doubt for the home Champions League game against Brest on November 26 and the following La Liga meeting with Las Palmas.
Yamal has scored six goals in his 15 games this season, while Lewandowski has 17 in all competitions and is currently the top goalscorer in La Liga.
Barcelona are sixth in the Champions League with nine points.
The Catalans are six points ahead of Real Madrid in La Liga but have played a game more than their title rivals.
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