Michael has given a good answer. It sounds like Sacks was a bit shocked by how well Robin Williams was able to imitate him. (The reference is to the move "The Awakening," which is about Oliver Sacks, and stars Robin Williams and Robert DeNiro. It's an excellent movie, by the way, if you haven't seen it.)

As Michael says, it means troubling. Another word you can use is discomfort. Sometimes, when people see videos of themselves or hear themselves in a recording, they feel uneasy, or uncomfortable. Imagine that type of feeling, but applied to Dr. Sacks.

"disconcerting" means "troubling". It is often used in the context of close likeness. e.g. "I saw a statue of Arnold Schwarzenegger holding a gun. It was such a good representation that I felt afraid - it was like the statue was alive and he was going to shoot me. The statue was disconcertingly similar to Arnold."