Both players joined Barca for fees well in excess of £100m over the past two transfer windows, although Dembele's career at the club so far has been adversely affected by injury.
The pair were both brought to Camp Nou in the wake of Neymar's blockbuster move to Paris Saint-Germain, but Ter Stegen does not believe that it is fair to expect an instant impact from either.
"Dembele didn't play very often because he was injured [and] Coutinho just came. We always need to give them time and be patient because they are young and they have some great years ahead," he told ESPN.
"Coutinho's very calm I think. He's very grounded and a good person. He can easily concentrate now on football, you see his quality and you need to give him time to adapt to the style of Barca, the way how we play and [it's] the same for Dembele.
"For Coutinho it is a bit easier as he is speaking Spanish already, Dembele needs to learn the language as soon as possible, to feel good in the locker room with all his colleagues. We want to give him a good feeling, so they can show everything they can."
Barcelona will face Chelsea in the first leg of their Champions League last-16 tie on Tuesday night, although Coutinho is ineligible for that match.