Saliba and Martinelli add to Arsenal's injury issues
France defender Saliba was forced off at half-time due to an unspecified injury in the Premier League leaders' win over Crystal Palace on Sunday.
Brazil forward Martinelli limped down the tunnel with a knock after coming on as a second-half substitute against Palace.
Arsenal boss Mikel Arteta, who revealed Declan Rice will be available to face Brighton after he was also injured against Palace, is unsure how long Saliba and Martinelli will be on the sidelines.
Losing two of his key players for any length of time would be a major setback as Arsenal try to win a first Premier League title since 2004 and a maiden Champions League crown.
The north Londoners are already without Martin Odegaard, Kai Havertz, Noni Madueke and Gabriel Jesus due to injuries.
"He's out. We are assessing him but he won't be involved in this match," Arteta said of Saliba on Tuesday.
"It looks as well Martinelli is going to be out. We have to do some more tests and see the extent of the injury but this game is going to be too early for him."
Martinelli's absence is another blow to Arteta's attacking options.
Viktor Gyokeres, signed from Sporting Lisbon in the summer, has started all 12 of Arsenal's Premier League and Champions League fixtures this term, scoring five goals.
Arteta, who last won a trophy with the 2020 FA Cup, is wary of becoming further depleted in attack and is likely to rest the Sweden striker against Brighton.
"In the front line, that's where we have more difficulties with Noni out, with Kai out, with Gabi Jesus out, with Gabi Martinelli out as well," Arteta said.
"That's the area where we have to be more careful, more conscious, because we don't have that many options.
"We are going to try to manage the load as good as possible, to make sure that the players are fresh, they can compete really well, and the team can maintain the level that it is showing."
Victory over Palace moved Arsenal four points clear at the top of the Premier League, with seven wins from nine top-flight matches.
Arteta is embracing the current optimism around the club after the frustration of finishing as Premier League runners-up for a third successive campaign last term.
"That's what we want," he said. "When our people are excited and you see that the team is competing at the level consistently, they are the right reasons to believe.
"If you want to win big trophies, you certainly have to install that feeling amongst the team, the club, and around it.
"We are in a really good position at the moment but we know how early it is and how difficult every game is."
