The 28-year-old began having treatment on his back towards the end of last season, which resulted in the centre-back missing the final two games of the campaign.
"I had some injections in my back and it was a hard month after them, but it seems to be getting better now," Agger told the club's official website. "I'm feeling the benefits, definitely.
"I'm feeling fine. [Pre-season training] has been hard, as it is every single year, but it's been good. We've done a lot of running but with the ball and it disguises all of the running a little bit. It's done in a nice way.
"Of course we're not there yet , but we're getting there. You can see the improvement every single day in training."