The 22-year-old suffered a nasty cut in the second half of last Monday's 6-0 win over Sheffield United and was not involved in the Gunners' 2-1 beating of Brentford on Saturday evening.
Martinelli was apparently seen on crutches after the latter game, but he was nevertheless called up to the Selecao selection for their imminent friendlies with England and Spain, which will be played on March 23 and 26 respectively.
However, the attacker did not take part in Monday's training session ahead of Arsenal's Champions League last-16 second leg with Porto at the Emirates on Tuesday night, which he is expected to miss.
Mikel Arteta remained coy on Martinelli's fitness levels during his pre-game press conference, but on Monday evening, Brazil head coach Dorival Junior confirmed that Martinelli had been cut from the ranks.
In his place, Porto attacker Galeno - who scored the winner for the Dragons in their first leg with Arsenal three weeks ago - has earned a spot in the team, but there is hope that Martinelli will recover in time for the Gunners' crunch clash with title rivals Manchester City on March 31.
The former Ituano starlet has endured a mixed season so far but has still come up with a respectable eight goals and five assists from 32 appearances, and six of his 10 direct involvements in the Premier League have come since the turn of the year.
Martinelli became an Olympic gold medallist with Brazil at the Tokyo 2020 Games and has now earned nine senior caps for the Selecao, scoring one goal and playing three times at the 2022 World Cup.
As well as Martinelli, Brazil have also lost Man City goalkeeper Ederson to the issue that he sustained in the weekend's 1-1 draw with Liverpool, which could keep the 30-year-old out for up to four weeks.
Ederson - who may therefore miss the visit of Arsenal on Easter Sunday - has been replaced by Leo Jardim of Vasco da Gama, while Paris Saint-Germain's Marquinhos has also pulled out, and his spot will be taken by Flamengo's Fabricio Bruno.
Martinelli will likely be one of two guaranteed absences for Arsenal on Tuesday evening, as Jurrien Timber is still working his way back from an ACL injury, but he is gradually increasing his workload in training.
Takehiro Tomiyasu was back in full practice on Monday after shaking off the knock which he brought back from the Asian Cup, but Arteta could not guarantee his availability for the second leg.
Failure to overturn their one-goal deficit would see Arsenal bow out of the Champions League at the last-16 stage for the eighth time in a row, but they have won their previous three home games against Porto by an aggregate score of 11-0.
The last time the Gunners advanced to the quarter-finals, they did so with a comeback success over the Portuguese side in the 2009-10 last 16, suffering a 2-1 defeat at the Estadio do Dragao but responding emphatically with a 5-0 win at the Emirates.