Unable to abduct upper left leg due to worsening muscle weakness. lower back pain x6 yrs. mri spine "not bad" mild disc herniation l4/l5/s1. now what?

Deconditioning: Is likey the issue. Pain can act as a hindrance to activity and once pain subsides the inactivity can cause disuse weakness. Work with a physical therapist to activate and strengthen those muscles. See your physician for an evaluation. To evaluate for muscle atrophy , reflexes to see if there are any neurological issues to be concerned about.

If muscle weakness: You should be evaluated by spine specialist. Either neurosurgery or orthopedic spine fellowship trained doctor who can relieve the pressure on the nerve by that disc. Assuming you have had conservative care, including physical therapy, possible injections by pain management, adjustment by chiropractor very briefly.