The injury caused Salah to ask to be substituted during Wednesday's 4-0 Champions League win over Galatasaray, after the 33-year-old shrugged off a missed penalty to score the final goal of the last-16 tie, which Liverpool won 4-1 on aggregate.
"He is not available for tomorrow. Good thing for Liverpool is we go to the international break. Bad news for Egypt, he can't go there," Slot told reporters, adding that Salah had a muscle issue.
"Mo has shown in the past he can recover faster than other players. He takes such good care of his body, he can be back earlier than others as history has shown. But it's only two weeks until we go again, so let's hope in that period of time he can be back."
Salah, who has netted four goals and provided four assists in his last nine appearances for Liverpool, was scheduled to travel to Barcelona, where Egypt are set to face Spain in a match that was originally scheduled to take place in Qatar.
The match was moved due to the conflict in the Middle East, the Egyptian Football Association said on Thursday.
Liverpool, who sit two points below fourth-placed Aston Villa in their Premier League title defence, are looking for their first win in three English top-flight games.
"We only have 60 hours of rest after putting in an enormous physical performance. Brighton has always been a team who want to play, bring the ball out from the back, and make it a very intense game," Slot added.