Injuries have been a persistent storyline for the 2015 first-round pick, as he's appeared in just 14 games over the past two seasons.
In 30 career games, Armstead has registered 50 tackles, six sacks and a forced fumble. Pro Football Focus gave him a grade of 70.1 last season, 49th among edge players.
Despite being injury-prone, expectations were high for the defensive end coming into the 2018 campaign.
"Arik's going to do a lot of things for us this year," 49ers defensive line coach Jeff Zgonina told Matt Maiocco of NBCSports.com in June. "I think he's more comfortable this year than last year. We tried him at a few spots last year, and I think he's really starting to feel comfortable at that big end spot and inside pass-rusher on pass downs. I think he has more of a defined role."
Armstead understands that he has a lot to prove in 2018.
"I've been hurt two years in a row, so that brings a lot of, 'What are you going to do?'" he said. "So definitely (I) have a lot to prove and look forward to having a healthy season, and I know what I'm capable of if I'm on the field."
The 49ers remain confident in his ability, picking up his fifth-year option this offseason. But Tuesday's injury isn't a great start to what is a very big season for the fourth-year edge-rusher.
While he's out of action, Cassius Marsh, Jeremiah Attaochu and Ronald Blair will see reps opposite Solomon Thomas at defensive end.