The 20-year-old shot to prominence at Borussia-Park last season as he helped his side finish fourth in the Bundesliga and qualify for the Champions League play-offs, while also making Joachim Low's provisional Mannschaft party for Euro 2012.
The young custodian was soon cut from the final 23-man squad, but links with the likes of Real Madrid and Barcelona continue to circulate, and Stiel, who kept goal for Monchengladbach between 2001 and 2004, believes Ter Stegen will only get better.
"Marc is better and has more quality than Manuel Neuer," the former Switzerland international told Spox. "It's quite clear - in the not-too-distant future, he will be Germany's No.1.
"Of course I trust him [to be ready for Madrid or Barca]. Whoever masters the Bundesliga is ready for any challenge."