A junior, Jack suffered a season-ending knee injury in practice two weeks ago, just a couple of days after his fourth-quarter interception locked up the Bruins' come-from-behind win over No. 19 BYU.
Mora said Jack thought it was in his best interests to focus on his rehabilitation full time.
"He's taking his chips and shoving them into the middle, and we hope he draws a good hand," Mora said.
"He doesn't see any point in staying in school. He's going to go and get some personal training and rehab."
Jack said later Tuesday in a statement via his Twitter account that he was ready to "fulfill a childhood dream" by jumping to the NFL.
Jack became a national sensation in 2013 when the coaching staff started using him as a situational running back. That year, he rushed for seven touchdowns and 267 yards on 38 carries.
But Jack always insisted he was a linebacker first and foremost. For his career, he totaled 178 tackles, 15 tackles for a loss and four interceptions.
Mora, who spent most of his career in the NFL before becoming UCLA's coach in 2012, said he thinks it's a risky decision because Jack played in only three games this season and might not have a clearly defined position yet at the next level.
"My personal opinion is, he's a tremendous football player and a tremendous athlete," Mora said. "I think it's risky to do this. Having been on that side, there's going to be a lot of speculation as to what he is and where he fits. And as I told Myles on Sunday, NFL teams are very, very conservative, and if there's any question whatsoever, they'll pass on you in a heartbeat. They're going to take the sure thing, and I explained that to him, but I feel like they already made their decision, and so, like I said, I hope it works out the best for him. But I, as a guy that spent half of my life in the NFL, I would move with great caution, and I would tell that to all of our players."
At 6-foot-1, 245 pounds, Jack played inside and outside linebacker, safety, cornerback and running back with the Bruins.
"If he played all year, I was thinking that we would [not have him next year], but when you only play in three games and that's all the tape they have of you your junior year. ... I've been in 25 draft rooms, and I've never seen a guy taken off of that ever," Mora said. "I worry about that for him. Like I said, I have a very personal relationship with Myles and his family, and his mom and his little brother Jaylen. Very close. He'll get himself in great shape and get ready and get rehabbed, perform well at the combine and perform well at his individual workouts. It's just a matter of if the scouts have enough game film to go off of, and they may, they may not. That's their decision, not mine. I just know that I've never see it, so it's kind of a new world for me."