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.
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.”
Carlo Ancelotti drop hints
Carlo Ancelotti has confirmed Arsenal defender Gabriel suffered an adductor injury while playing for Brazil.
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.
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.
It remains to be seen whether Gabriel will be fit to feature against Spurs, with Ancelotti providing a cautious update on the Gunners star after Brazil beat Senegal 2-0 in north London.
‘Bad? I don’t know,’ the former Real Madrid manager said. ‘He had an injury in his adductor. The medical staff has to check tomorrow.
‘We are really sorry for this, really disappointed. When a player has an injury, you hope they can recover well and soon.’
Brazil beat Senegal 2-0 at the Emirates Stadium thanks to goals from Chelsea winger Estevao Willian and Manchester United midfielder Casemiro.
Gabriel is not the only cause for concern among an Arsenal fanbase hoping to see their side win the league title for the first time since 2004.
Italy boss Gennaro Gattuso has confirmed fellow Gunners defender Riccardo Calafiori has left the international camp following ‘a few issues’.
Calafiori missed Italy’s World Cup qualifying win over Moldova on Thursday
‘We tried Calafiori, he had a few issues,’ Gattuso told Sky Sports ahead of Italy’s match against Norway on Sunday.
‘I thank him for his commitment. He stayed here for a week, he could have played perhaps, but it wouldn’t have been fair to him or Arsenal.’
As well as starring in the most solid defence in Europe, Gabriel has developed into a real offensive threat over the past two seasons.
‘I think you can make an argument that he could be seen as the most influential player in the Premier League right now,’ Liverpool legend Jamie Carragher said of Gabriel last month.
‘When you think of the clean sheets and the defensive record Arsenal have and also his threat at set-pieces. He’s as influential as anyone, maybe even Erling Haaland.’
Arsenal captain Martin Odegaard, summer signing Noni Madueke and first-choice striker Viktor Gyokeres are all also recovering from injury problems.