The 46-year-old has voiced his opinion that a move to Spain could hurt his compatriot's career at this stage and has advised them to pursue Thibaut Courtois instead
The Catalans are in the market for a new shot-stopper after Victor Valdes signalled his intention to leave the club, and the 21-year-old is supposedly one of the leading candidates for the No.1 jersey at Camp Nou.
However, Illgner feels Ter Stegen is not the right candidate as he has advised Barcelona to go for Chelsea's Thibaut Courtois, who has spent the last two season on loan at Atletico Madrid, instead.
"He [Ter Stegen] is incredibly talented, but also still very young. One should realise that a transfer at his age could affect a career in two ways, both positive and negative," the former Madrid shot stopper told Spox.
"If you make a mistake at such a big club and get off to a bad start, it would leave a big mark on his career.
"For me, someone else is the ideal candidate and that's Thibaut Courtois, currently on loan at Atletico Madrid from Chelsea."
Ter Stegen has a contract with Gladbach until June 2015, while Courtois' Chelsea deal runs until the summer of 2016.