Media speculation had the Argentine midfielder almost certainly in a Barcelona jersey after news that he was growing tired of life in England but after a deal fell through, he remains a Reds player.
Now Benitez believes that Mascherano will give his all for Liverpool.
"Javier knows he is staying," said Benitez. "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 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.
"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."