In a clash lacking quality throughout, the Baggies should have gone in at the break a goal up but saw Karlan Grant miss a penalty.
Ismael's side did open the scoring shortly after the interval with Kyle Bartley producing an excellent header to nod home a Conor Townsend delivery.
But Millwall pulled level just minutes later when Daniel Ballard scored an equally impressive header.
The result sent Albion top of the Championship with Ismael's side now the only unbeaten team in the division.
But they have now struggled to break down Peterborough and Millwall in their last two outings - with both sides sitting deep and allowing Albion to have more of the ball.
"We need to learn from this and adapt to when the opponent does not want to take any risks and wants to take the intensity out of the game," he said.
"Today is exactly what we will face over the new few months.
"The opponent will come here, they won’t want to take any risks and they will want to take the intensity out of the game.
“Nevertheless, we had the possibility to score from the penalty and after that we came back very strongly, scoring the goal just after half-time.
"But today, the big difference is we made too many mistakes, in the backline especially - with the back passes to the keeper.
"It put us in danger and we gave the opponent the feeling they could come back into the game.
"They got corners, free-kicks, throw-ins. And you know against Millwall that is a massive part of their game, you need to avoid that but we made too many mistakes.
“We will learn from this and adjust but we need to put more energy to address that intensity.
“We have to be more ruthless in our back line – that was our strength in the first five games so we need to get back to that and dominate our opponent.”