Elimination of the knight: Direct or Indirect? Between 18. Na2 and 18. Rac1, which is more favorable for white?
IM Silman gives this position as problem 3 in The reassess your chess workbook. He recommends 18. Na2:


  Short played the surprising and strong 18. Na2!!. This idea ... shows that White considers the b4-Knight to be more important than its brother on e4. The reason is simple: the b4-Knight is the main guardian of Black's weak points on a6, c6 and d5.


IM Silman does not consider 18. Rac1 in his analysis.