Grujic is now rated in the €18m class thanks to his form this season.
He told BILD: "18 million is better than five (laughs). It shows that you have improved a lot in a short time. Most of all my self-confidence has increased due to the match practice. My defensive play has also improved a lot because of the games."
Asked about Liverpool's plans for him, Grujic added: "The last time we talked was when I was in England because of my injury (in December 2018).
"We will see what happens next season, now I am concentrating on the remaining games this season with Hertha."