"Martin is a top, top player. I was surprised when he came just how good he was," Agger told official publication LFC Magazine.
"I have to be honest and say I don't know much about the Russian Premier League, so I did not really know what to expect when Martin came to the club.
"But he's a really good player and, just as importantly, he's a really good guy to have in the dressing room too. He's a guy who's good to talk to away from the pitch and who I admire a lot because of the way he plays on it.
"If you play all the time, if you get the chance to play week in, week out, then you must be a good player.
"I don't care if Martin and I are rivals for a starting place, he's still a friend of mine and I'll still tell the world how highly I rate him.
"He's an excellent defender and will keep getting better.
"We are competing with each other, maybe, to play in the team but it's not a rivalry. Both of us play and train to the best of our ability and then the manager makes the decision."