There was more concern reported Monday about UCLA linebacker Myles Jack’s knee injury that limited him to just three games last season.

NFL source calls Myles Jack "a time bomb" whose knee could give him several good years, or not. Bone and cartilage starting to break away. — Les Bowen (@LesBowen) April 18, 2016

Jack, considered one of the best defensive prospects available in this year’s NFL Draft, had his meniscus surgically repaired in September. His recovery received mixed reviews from the medical re-checks done in Indianapolis, Ind. last week.

Jack participated in agility drills at his pro day last month, of which 49ers general manager Trent Baalke attended. Jack posted an impressive 40-inch mark in the vertical jump, but has not run a 40-yard dash, nor does he plan to before the start of the draft.

The source compared Myles Jack's knee situation Jonathan Vilma. Vilma made the Pro Bowl twice, won Super Bowl, but only had 6-7 healthy yrs. — Les Bowen (@LesBowen) April 18, 2016

Jack is a candidate for San Francisco at pick No. 7 overall April 28 to potentially pair next to NaVorro Bowman in the middle of its 3-4 defense. Jack is athletically gifted and would complement Bowman well while the four-time All-Pro continues to work his way back to 100 percent after missing all of 2014 to recover from his gruesome knee injury suffered in the previous conference title game in Seattle.

If fully healthy, Jack might be the best possible option for the 49ers, who have needs up and down the roster, including at inside linebacker, defensive line, offensive line and pass rusher.

After losing former All-Pro Patrick Willis to sudden retirement in 2015, San Francisco tried to fill the void with Michael Wilhoite and reserve Gerald Hodges last season, but were unable to replicate Willis’ impact along side Bowman.

Jack, presumably, would give the 49ers their best chance at replicating the success they had with the Willis and Bowman pairing, if his knee is healthy enough by the time training camp comes around in late July.