The 17-year-old picked up the problem in the 5-2 Champions League win over Red Star Belgrade in midweek.
His involvement with Spain in upcoming UEFA Nations League games against Denmark and Switzerland on Nov. 15 and Nov. 18 respectively is also in doubt.
“Yamal sustained a severe contusion in Wednesday’s game against Red Star Belgrade,” Barça said in a statement.
“Over the past few days, he has undergone treatment, but the discomfort has yet to subside. His recovery will determine his availability.”
After Sunday’s game in San Sebastián, Barça are not in action again until Nov. 23, when they travel to Celta Vigo.
Yamal has played a key role in helping Barça to the top of the LaLiga table so far this season, scoring six goals and setting up eight more in 16 appearances in all competitions.
