Mustaine's damage was probably less profound, but I think the fine control needed for the kind of thing he does is rather more extreme than Mitchell's.
ETA: here's a lousy article on it, from a music industry rag. He talks about it in much more detail in his autobiography. Below is a quote from https://blabbermouth.net/news/megadeth-s-dave-mustaine-how-i...
On January 7th, 2002, while at the [drug rehab] hospital, I sat on a chair which I hung my arm over the back of. The hard edge along the top of the seat back cut off the circulation to my radial ulnar nerve. After approximately two hours I woke up and my left hand was numb. I went to the nurse's station and they said it was the hair-tie I had on my wrist. I wish. I had to go into town to see a specialist and he said that I would be lucky if I ever gained even 80% of the use of my arm again. This was unacceptable for me, so I left the rehab, against medical advice and when home to Scottsdale, Az. to get my arm checked out by a city doctor. My Dr. Rahj Singh, a expert in nerve damage, spinal damage, etc. said that I may get 100% use of my arm, but that I would never play the same. He then prescribed the braces you see [here]: Photo#1, Photo#2, Photo#3. I then proceeded to Nathan Koch for physical therapy for 4 months of sessions, three times a week, 1-1.5 hours a day. After I finally got my feeling back in my hand, I realized that I could not even hold a feather in that hand and started a grueling 1-year weight-training program. 13 months after I hurt myself, a personal assistant that had worked for me died in hospice of drug damage, and I was asked to play. It was the first time I had held a guitar since November 17, 2001. Since then, I have completely healed and started taking lessons intermittently to re-learn my trade. After an additional 5 months I decided that I was going to play again, but that is another story.