SANTA CLARA, Calif. -- After their 2020 season was ravaged by injury, the San Francisco 49ers hoped that 2021 would bring better health. They're not off to a promising start.

During an organized team activity Monday, the Niners lost two players to what will almost certainly be season-ending injuries more than three months before the 2021 campaign begins. Offensive lineman Justin Skule suffered a torn anterior cruciate ligament in his knee, and safety Tarvarius Moore tore his Achilles, according to a team spokesperson.

While neither Skule nor Moore has been an established starter for the Niners, both have played prominent roles in recent seasons.

Skule has played in 31 regular-season games over the past two years with 12 starts at tackle and guard. He was expected to compete with Shon Coleman for the swing tackle job in training camp before the injury.

Moore had played a similar role in the secondary, appearing in all 48 regular-season games over the past three seasons with 13 starts. He had 49 tackles, a forced fumble and a fumble recovery in 2020 when he started eight games. He was expected to compete for the starting job at strong safety before Monday's injury.

Without Moore, the Niners have a group including veterans Jaquiski Tartt, Tavon Wilson, Marcell Harris and Tony Jefferson, who signed on Monday, and rookie Talanoa Hufanga to compete for spots on the safety depth chart.

Skule and Moore are the latest in a long run of disappointing injuries to the 49ers. In 2020, the Niners lost 161.6 games to injury, the second most of any team in the past 20 years, according to Football Outsiders' adjusted games lost metric (which also factored in players missing game for COVID-19 reasons).

Many of those injuries affected some of the Niners' most important players, as quarterback Jimmy Garoppolo, tight end George Kittle, ends Nick Bosa and Dee Ford, receiver Deebo Samuel, cornerback Richard Sherman and running back Raheem Mostert combined to play in 37 of a possible 112 games (33%).

After the season, general manager John Lynch and coach Kyle Shanahan vowed to "look at everything" to try to get and stay healthier in 2021. That included an increased emphasis on durability among the draft class and in outside free agents they acquired.

"What I've learned with some of our luck here, especially last year ... when too many of those add up, it's hard to compete," Shanahan said in May. "And I think that hit us harder than anything last year. That hit us before COVID, and that's something we can't do again. So, we're not saying that we'll never take a risk again or anything, but we definitely wanted to make a point because of what's happened the last couple of years, at least to try to avoid that."