The German described his teammate as "world class" but was quick to point out that it may take him several more seasons to justify comparisons to the Portugal international.
"This season is unbelievable. He's of course in that region but the biggest difference is Ronaldo did this over so many years. It's too early to put Mo with him," Karius told Sky Sports News ahead of this weekend's Champions League final between Liverpool and Real.
"I don't want to take anything away from Mo - he's world class this season. But Ronaldo showed over so many years to be up there with the best, you have to prove it season over season.
"I hope he can do it. I wish he can do it. He's still really young and has a lot of space to improve even more."
Both Ronaldo and Salah have scored 44 goals for their clubs this season.