The Reds stopper came under fire following Sunday's International Champions Cup clash with Borussia Dortmund after mishandling a Christian Pulisic shot into the path of Jacob Bruun Larsen for a tap-in.
Karius's error followed on from a mistake against Tranmere Rovers earlier this pre-season, the German fumbling a free kick to eventual scorer Jonathan Smith, on top of his errors in the Champions League final defeat against Real Madrid.
Casillas, who won a World Cup and two European Championships with La Roja as well as multiple trophies with Real Madrid, has had enough of the abuse which has been sent the way of Karius.
"Will this attack Loris Karius ever end? I'm talking about him like so many other goalkeepers," the 37-year-old Porto player wrote on social media.
"There are many more serious problems in the world - fuck! Leave the kid alone! He's also a person. As we all are!"