Bell is going nowhere. Pietersen will play shots and score runs, so the longer he's out there, the more steam he builds up and the more he'll know about his innings and the conditions. Bell has the temperament, but lacks the equipment to handle the task. He's simply not able to manufacture his own shots and impose his innings. A grafter should still pick up singles by cunning strokes and expertly soft hands, nudging the field so that a gap might open. Even a big strokemaker is as hungry for ones and twos as fours.
Bell is waiting for the most perfect ball to score from and Glenn McGrath is just not going to offer him that, even Brett Lee is going to make him work for a boundary. If he's offered something too wide or too straight, the onus is on him to make it a bad ball. Eventually, Warne will come on and Bell will be a perfect target, slow cooked but underdone.