Karius was blamed for Borussia Dortmund’s third goal in their 3-1 victory over the Reds, having made a mistake at Tranmere earlier this month.
In May, the German made two high-profile errors in the Champions League final defeat by Real Madrid.
“Stay strong Karius, it has happened to the best of players,” Salah Wrote.
Stay strong Karius, it has happened to the best of players. Ignore those who hate.@LorisKarius
— Mohamed Salah (@MoSalah) July 23, 2018
“To those who take joy in seeing other people fail or suffer, I feel for you,” he wrote.
“Whatever it is that’s happening in your life to hold this much anger and hate, I pray that it passes and good things come to you.”
Liverpool manager Jurgen Klopp has defended Karius, but warned the goalkeeper will continue to be criticised until “he has a few fantastic games”.