Gabriel was forced off while playing for Brazil on Saturday
Arsenal Football Newsletter
The Arsenal defender was forced off in the second half as Brazil beat Senegal 2-0 at Emirates Stadium on Saturday.
His fitness is a major concern for Mikel Arteta ahead of the north London derby against Tottenham.
Gabriel has been a key player for Arsenal at both ends of the pitch this season.
Gabriel received treatment to his right thigh before being substituted
He has been a rock in the defence that has conceded just five goals in 11 Premier League games, as well as being their biggest threat in the air from attacking set-pieces.
Gabriel went down just before the hour against Senegal and received treatment before being substituted.
Arsenal face Spurs at Emirates Stadium on Sunday and also face Chelsea in a huge Premier League clash later this month.
“Bad? I don’t know,” said Brazil manager Ancelotti.
“He had a problem with his adductor. We are really sorry for this.”
Arteta is also sweating on the fitness of Riccardo Calafiori after he withdrew from the Italy squad with a hip problem.
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Arsenal have already been stretched by injuries this season, with Viktor Gyokeres, Martin Odegaard, Noni Madueke, Gabriel Martinelli and Kai Havertz all sidelined before the international break, as well as long-term absentee Gabriel Jesus.
Norway coach Stale Solbakken has hinted that Odegaard is unlikely to return in time to face Spurs.
“It is steady,” Solbakken said of Odegaard’s injury recovery this week. “It is going in a steady direction but he is some distance away.”