The Spanish giants have been constantly linked with a swoop for the Argentina midfielder but Benitez has told Mascherano he is going nowhere.
Liverpool sold Xabi Alonso to Real Madrid for £30m last week and Benitez is keen not to lose two star players in the same transfer window.
"Javier knows he is staying," the Liverpool boss said. "He has accepted his future is here and that he is going to be a key player for us once more.
"Mascherano is now fully focused on us and what we might achieve together this season, I can guarantee that 100%.
"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.
"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."
Mascherano's performances since return from international duty have been below par, but Benitez is confident his performance in the 2-1 pre-season defeat to Atletico Madrid on Saturday shows he will once again be a key player this season.
"He played much better against Atletico than he has before," added Benitez. "I am confident he is more focused now. The last week has changed the situation a lot and he is playing much better."