Depends on how you define "supportive care," I guess. You can prevent many infections with cleanliness, knowledge of wound care, and antiseptics like alcohol; set bones, as Heinlein said; stitch wounds; do surgery, again with just alcohol and low-tech tools; diagnose illnesses; treat the ones that can be treated with lifestyle changes, diet alterations, environment alterations, first aid, etc); do physical therapy; prevent many epidemic illnesses from beginning or spreading; etc. An apocalypse doctor would lose a lot of patients, but also do a lot of good - largely depending on how much the community paid attention, of course.