What does the doctor mean when he tells Macbeth "Therein the patient Must minister to himself?”
It is only relatively recently that we have stopped using "he" to refer to the universal human. When the doctor says this, he is more likely offering the audience a truism than commenting on Macbeth's guilt. Although he knows of it, it would be madness for him to even hint that Macbeth is guilty of anything untoward, for to do so would be to incur a risk the doctor seems unlikely to take (he wants to be as far away from there as possible, and fast).
I agree with tthakar, in that the physician is suggesting that the remedy is outside of his area of expertise. The doctor knows that it is the disruption in her mind that causes her unrest. He says earlier that she is much more in need of the divine (a priest) than a physician.The doctor's comment is somewhat ironic and sad given that this would have been Lady Macbeth's advice to Macbeth only a few scenes earlier. Through Lady Macbeth's illness, Shakespeare shows the depth of destruction wreaked in the heart and soul of man (and woman) by guilt.
It is interesting that the Doctor answers Macbeth's question regarding his wife with the male pronoun "him." The Doctor should say that Lady Macbeth needs to minister to herself, but because he answers with "himself" it proves that the Doctor is aware that they are both guilty. Macbeth is too busy waging war to care much however.