The 22-year-old Spain international, who scored twice in Barcelona's 6-0 win at Valencia this month, picked up the injury late in Sunday's game after coming on as a second-half substitute.
"The player Fermin Lopez has an iliopsoas muscle injury in his left leg. He will be out of action for around three weeks," Barcelona posted on X.
Champions Barcelona, who visit Oviedo on Thursday, trail leaders Real Madrid by two points.