The Dane is set to face his former loan club today.
He told the club's website, "I enjoyed being there and playing football there. Everything was so local. You had Dusseldorf which was a big city 20 minutes away. I lived in Monchengladbach in an area where some of the players lived. Because Monchengladbach is not the biggest city, so much is about football. When you go to the shop, you'd see the players' faces on different things, you'd see flags out the cars, flags on every little shop.
"At the same time, when you went out with the players, people were so nice and you could still be private. I enjoyed the whole experience of being there.
"I could feel in the first couple of games it was big step because I had only played youth team football and a handful of cup games for Chelsea before that. You gain a lot from training with the first team at Chelsea, but game-time is important, and for me stepping up from playing in the Youth Cup or the UEFA Youth League, it was a big step going directly into the team and playing in the Bundesliga.
"The football on the pitch was quick, a lot of back and forth. I learned a lot from that. You'd be put in difficult situations as a back four, especially as it was so new to me at the time."