The former Germany defender watched the youngster closely during Dortmund's draw at St Pauli and admitted he expected more.
"I was at the game at St Pauli and saw him live," Helmer told Football Presse.
"He only played one half. But I was not particularly impressed. The whole team was very passive and he was as well."
Helmer felt Bellingham failed to influence the game.
"He did not have any special impact and there were no real good actions going forward."
While he praised the teenager's earlier showing at the Club World Cup, Helmer stopped short of full approval.
"I am not 100 percent convinced. Because of his name he will be watched very closely. I know how difficult that is."
And the key doubt remains.
"I do not know if he is the player who will really help Borussia Dortmund go very far internationally."
Still, Helmer urged patience.
"You have to give him time. He is still very young. The Bundesliga is different and Dortmund play differently."
For now, the verdict is cautious.
"If he is tactically clever, which I think he can be, then he can become an important factor."
Not a rejection.
But not blind faith either.