Atticus makes a point that no one got a doctor for Mayella because they kept saying that she was injured. If she was so badly injured, there should have been a doctor called. Atticus tells Mr. Ewelll that "folks were doing a lot of running that night” (ch 17).
Let's see, you say you ran to the house, you ran to the window, you ran inside, you ran to Mayella, you ran for Mr. Tate. Did you, during all this running, run for a doctor?" (ch 17)
Ewell says he does not need a doctor because he could see what happened, and he was concerned about her condition because he saw who did it. None of these answers are related to the actual concern for Mayella’s health or pain.
As a member of the jury, I probably would have wondered about the fact that no one called Mayella a doctor. I would worry about what happened to Mayella, and begin to think that maybe she was not really raped after all. This is what Atticus wants the jury to think. He wants them do doubt Mr. Ewell's story, and see that he is not the best father.
Although Atticus is an excellent lawyer, he is up against impossible odds. Members of the jury will feel that no matter what happened, it is the word of a white woman against that of a black man. That will be enough for them.