He told the Portsmouth News: "The defence has to learn. Hermann is getting better and better, he's got enthusiasm but needs buckets of work.
"He played centre half for Charlton last year now he's left back for us. I'm glad he's playing at left-back for Iceland at the moment, that will help.
"Sylvain as well, he's got to learn about Sol and what he does. It's about the balance between who's going to attack the ball.
"There is still more to be done."