Gregory Louis Bonin
Greg Bonin was a major league umpire from 1984, when he worked his first National League game until 2001, by which time he was a part of the consolidated major league staff. He most notably worked the 1991 All-Star Game and 1998 NLCS. He had to retire due to repeated concussions, including being hit in the head by a Charlie Hayes swing and a Wil Cordero foul tip. Bonin was still battling memory and concentration issues years later.
Sources include USA Today