LOS ANGELES - Myles Jack, No. 9 UCLA's versatile linebacker and running back, will miss the rest of the season due to a knee injury, coach Jim Mora announced Wednesday.

Jack hurt his knee during a non-contact drill near the end of Tuesday's practice, and he had surgery that evening. The junior is the third key Bruins defensive starter lost for the season with injuries this month, joining defensive lineman Eddie Vanderdoes and cornerback Fabian Moreau.

Jack might be the biggest loss of all for the Bruins (3-0), whose national title aspirations have been seriously damaged even without a loss heading into the start of Pac-12 play Saturday at No. 16 Arizona.

With his two-way skills and incredible athleticism, Jack was the most prominent player on the Bruins' roster along with freshman quarterback Josh Rosen. Jack is one of the nation's top linebackers, finishing second on the roster last season with 88 tackles while showing off his NFL-ready playmaking acumen.

Mora and defensive coordinator Tom Bradley reacted dispassionately to Jack's absence after UCLA's early-morning practice on campus, perhaps concealing their concern with confidence.

"We've got some depth," Bradley, the first-year coordinator, told reporters. "We've got to develop it. I know the guys that are going to come in are excited for the opportunity. ... We have some people with some skill levels that can do some other things. They'll get some reps this week, and they've adjusted to it pretty well."

Jack has 15 tackles in UCLA's first three games this season. He also lines up in the slot to cover receivers, showcasing his exceptional speed for a linebacker. He made the victory-clinching interception late in UCLA's win over BYU last weekend.

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He also carries the ball occasionally for the Bruins, particularly excelling in goal-line and short-yardage situations. He scored seven rushing touchdowns as a freshman -- averaging 7.0 yards per carry -- and three more last season.

He had one of the most memorable offensive games of his career at Arizona Stadium in 2013, rushing for 120 yards on just six carries, including a 66-yard touchdown run. He was the Pac-12's offensive and defensive freshman of the year.

Jack has carried the ball just twice this season, but he scored his 11th career touchdown in the season opener against Virginia. He was likely to play an increasing role on offense as the season wore on.

UCLA could be reeling in Tucson after major injuries to arguably the best player at every level of its defense. Vanderdoes played alongside Kenny Clark as the heart of the Bruins' defensive line, while Moreau is a senior and UCLA's top coverage cornerback.

Vanderdoes, a junior, injured his knee in the season opener against Virginia. Moreau hurt his foot early in the Bruins' 24-23 victory over BYU.

There's even the possibility that Bruins fans will never see Vanderdoes and Jack in uniform again. Both juniors are considered solid NFL prospects, and they could enter the draft next spring.