Salah, the league's joint-top scorer this season with seven goals, has a contract with Liverpool until June 2023.
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"If you ask me, I would love to stay until the last day of my football career. But I can't say much about that, it's not in my hands," Salah, 29, told Sky Sports.
"It depends on what the club want, not on me. At the moment I can't see myself ever playing against Liverpool. That would make me sad.
"It's hard, I don't want to talk about it, but it would make me really sad... but let's see what will happen in the future."
"It's one of the top games in the world, in England especially," Salah said of the upcoming match against United. "For big games you have to give everything, you have to be focused."