The 25-year-old defender has produced some impressive displays of late and the Reds boss feels an injury-free Agger can have a massive impact on the side.
"Daniel is a player who will be even better in the future because he is eager to learn and working hard in training all the time," said Benitez.
"You saw the other day that he is a defender who likes to play the ball and he is going to become a very important player for us in the future.
"When you want to play well as team, it is important you start with the defenders and to do that, you need someone who can defend but also you need to rely on them in possession."
He added: "For this club to progress, we need important players like him.
"He can pass it short, he can play it long, he can switch play and can score goals. His form is very good.
"We will have to keep working with him with his fitness.
"He has had some problems this year and also had a virus. It has been difficult in the last month as he played too many games."
The Reds face a tough Europa League test away to Benfica on Thursday before travelling to Birmingham on Easter Sunday.
Benitez acknowledges that it could be a crucial week in the club's season and is hopeful the attacking verve shown in the 3-0 triumph over Sunderland is just the start of things to come.
"Everybody wants to see attacking football - even the manager!" said Benitez. "If we can keep progressing in the Europa League and finish well in the Premier League, it will be easy to talk about the future.
"Liverpool is a massive club and there are lots of players thinking about coming here. That's why it is important for the future that we perform now.
"The effort has been good but we need to keep pushing. We know we have not done well this season and that has had an impact on our confidence.
"But you cannot change what has happened in the past. The only thing we can do is keep moving forward and try our best until the end of the season - that is the message."