McDonald, a starter the last two seasons and a freshman All-American in 2011, was third on the team with 79 tackles last season and tied a school record with three interceptions in a game as a freshman.
McDonald was thought of as one of the team leaders, and Mora selected him as one of two representatives for Pac-12 media day prior to last season.
"It's disappointing to me," Mora said. "I'm disappointed for Tevin."
McDonald also was suspended from UCLA's Holiday Bowl appearance in December for a violation of team rules. The Los Angeles Times reported at the time that McDonald had failed three drug tests. UCLA's policy is that a player who fails three drug tests is suspended for a game and failing any subsequent drug tests would result in the loss of scholarship.
Mora, citing legal reasons, could not reveal the nature of the violation that resulted in McDonald's dismissal or if it was related to his Holiday Bowl suspension. He simply expressed sadness over losing a player who he has known for a long time.
McDonald's father, Tim, played safety for the San Francisco 49ers when Mora was the defensive backs coach for the 49ers. He said Tevin and his brother T.J., a USC standout the past four seasons, used to hang around practices and the locker room.
"I've known Tevin since he was a young boy," Mora said. "I probably met him when he was 5. So just on a personal level, it's disappointing. I care about him and I hope that everything turns out well for him."
Mora hinted that McDonald would probably try to transfer to another school.
""He's got a great family, he's got great parents and I know that they'll find the right avenues for him to have success," Mora said. "He's a good kid."
Losing McDonald leaves an already thin UCLA secondary with another question mark. McDonald was slated to start, but now sophomore Randall Goforth is the only returning player who started at any secondary position last season.
Dietrich Riley, a starter at safety two years ago, is slated to return from neck fusion surgery. The Bruins have signed a top-notch class of secondary players, including Priest Willis and Tahaan Goodman, who were both ranked among the top 10 safeties in the Class of 2013.
"We're really light at that safety position," Mora said. "We've got reinforcements coming in the fall with the four really good players that we signed, but they're not here yet. I think the one interesting thing will be to see how Dietrich is. He's been cleared."