He said: "Mascherano is now fully focused on us and what we might achieve together this season, I can guarantee that 100 per cent.
"We've spoken about his situation before and we spoke again. Before, it was difficult and he could see Xabi Alonso was on his way.
"But all that is behind him now. He has accepted his future is here and that he is going to be a key player for us once more."
Mascherano knew he would remain on Merseyside as soon as Alonso moved to Real Madrid for £30million.
And Benitez added: "Javier knows he is staying, he's clear about that in his own mind.
"There has been a significant improvement in his game over the past 10 days. He has been a lot more focused, and we are seeing the real Mascherano again."