The German keeper spent five years (2005-2010) in the club's youth and Reserves ranks.
Ferguson said: "Ron-Robert has come on leaps and bounds since joining Hannover and is now a regular in the German national set up. He was always a hard worker and a he's a really nice lad as well. It was unfortunate for him when he was here as he got an injury in the Reserves and was out for about eight months.
"When he came back, Ben Amos had jumped ahead of him in the pecking order. I’m looking forward to seeing him again when we play Hannover 96."