No. 9 UCLA now faces the remainder of the season without three of its best defensive players.
The Bruins announced Tuesday that All-Pac-12 cornerback Fabian Moreau had suffered a season-ending Lisfranc foot injury. UCLA, which visits No. 16 Arizona on Saturday, previously lost defensive end Eddie Vanderdoes to a knee injury after the season opener against Virginia.
Jack, who injured his knee during Tuesday's practice, was the Bruins' third-leading tackler. A two-way player, he also had scored a touchdown as a running back.
The 6-foot-1, 245-pound Jack is widely seen as a candidate to enter the NFL draft this spring as a potential first-round selection.
With his two-way skills and 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.
"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 unusual speed for a linebacker. He made the victory-clinching interception late in UCLA's win over BYU last weekend.
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 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 touchdown in the season opener against Virginia. He was likely to play an increased role on offense as the season wore on.
UCLA could be reeling Saturday 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.
Information from The Associated Press was used in this report.