Lee will need to rest for 4-6 weeks, but he might be able to handle a dozen overs in the final innings if they need.
Even though he's only got one wicket in the series, he's the spearhead. He's usually near the bowler, always having a word to his cohorts. Missing Clark was already a painful blow, but in terms of structure, this is perhaps worse. Although he's only played about one season's worth of first class games more than Stuart Clark, most of Lee's first class experience has come at international level and so they lose that experience that Lee has garnered under the wing of McGrath or Gillespie; just invaluable stuff. By comparison, Mitchell Johnson is less than half as experienced as Stuart Clark and Siddle's first class career is less than half that again. I don't know that simply picking a good bowler is enough. The player who comes in should be one that can rally the attack, not simply contribute to it.