Mane suffered a hamstring injury on international duty with Senegal and is unlikely to return to fitness until November.
The pace of the former Southampton attacker is a key outlet for Liverpool, who will instead look to the likes of Salah to offer a threat on the break when United visit Anfield on Saturday.
“I texted him [Mane] when he was at the national [team] and then when he came back. He told me then he was injured so that’s bad news for everyone,” Salah told the Liverpool Echo.
“But that’s football and anything can happen. Now we have to focus on the game and keep moving forward.
“It’s a big game on Saturday, everyone in the world will watch the game. It’s a big game for us too because we have to keep fighting and improve our results. I hope we can get the win.
“We always fight in each game, but this game is a big game and can make a difference for us. We have to improve our place in the table. We have to fight and try to win the game.”
Liverpool have won just once in their last four Premier League matches and sit seventh in the table, seven points behind joint leaders United and Manchester City.