In the world of agriculture, as with popular medicine, the theory is to treat the symptom (in this case insect invasions) and to hope that by the time fix becomes ineffective we have something else figured out to spray, engineer or swallow to re-bandaid the problem.
Ancelotti used half-time to replace the ineffective John Mikel Obi with first-choice holding midfielder Michael Essien and the move seemed to give Chelsea added impetus.