Maybe the Wizard doesn't know any healing magic, and that's why they need the Doctor. Honestly, as long as the Doctor's alone, he'll probably be easier to defeat than the Wizard. I mean, what's he going to do? Poison us? Send mad experiments at us?

[Never mind that those are actually very valid options!!]

I've dealt with some doctors in my time; once you fight them one-on-one they tend to go down in one blow.