The Portuguese international replaced Rohan Ricketts on 76 minutes against West Ham and the striker played in tandem with Bobby Zamora in what looked a promising pairing with Robbie Keane dropping deep.
Zamora got off the mark to seal our Carling Cup passage to a home tie with Manchester City and David suspects a Postiga goal will not be too far behind.
"It will come for him - he is very strong player for his age," he said afterwards. "He has got good things in his game, but it takes a while to settle.
"We give everyone time and we're not selling anybody. We certainly not selling anybody we don't want to sell - we are not in trouble."
David Pleat has backed Helder Postiga to soon get among the goals and completely refutes reports of a January sale.