LOS ANGELES - Injured UCLA linebacker Myles Jack has dropped out of school, and he plans to enter the NFL draft next year.

UCLA coach Jim Mora expressed trepidation about the injured Jack's decision when he announced it Tuesday.

"He's taking his chips and he's shoving them into the middle," Mora told reporters after practice with the 20th-ranked Bruins, who have a bye this week. "We hope that he draws a good hand. At least I do."

Jack is out for the rest of his junior season after tearing a ligament in his knee during a non-contact drill in practice last month. He was a starting linebacker and a frequent running back for the Bruins during his first two seasons, showcasing his electrifying athleticism on both sides of the ball.

But Jack will leave after just 29 games at UCLA. Mora has known Jack since the player was 12 years old, and the coach expressed support for Jack even while acknowledging serious concerns about the decision.

"I think it's very risky to do this," Mora said. "Having been on that side, there's going to be a lot of speculation as to exactly what he is and where he fits. As I told Myles on Sunday, NFL teams are very, very conservative. If there's any question whatsoever, they'll pass on you in a heartbeat. They're going to take the sure thing. I explained that to him, but I feel like they had already made their decision."

Mora was an NFL head coach in Atlanta and in Seattle, where Jack grew up. He has close ties with Jack's family in Bellevue, Washington.

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The decision ends a short UCLA career for Jack, who played two full seasons.

He was the Pac-12's freshman of the year on offense and defense in 2013, and he maintained his spot as one of the conference's most interesting and exciting players last year. He made 15 tackles and a game-clinching interception in three games this season, but got hurt before Pac-12 play began.

"When you only play in three games, and that's all the tape they have on you your junior year, I've been in 25 draft rooms and I've never seen a guy taken off that," Mora said. "Never. I worry about that for him. I hope it all works out the way he plans for it to work out, I really do."

Jack could need six months to recover fully from his injury and subsequent surgery. The linebacker decided to rehabilitate his leg at an out-of-state training facility away from the distractions of schoolwork and UCLA campus life, Mora said.

"He doesn't see any point in staying in school," Mora said.

The Bruins (4-1, 1-1 Pac-12) lost three key starters from their defense in the first quarter of the season. Defensive lineman Eddie Vanderdoes and cornerback Fabian Moreau joined Jack with season-ending injuries, and their absences finally showed up in UCLA's home loss to Arizona State last weekend.