On Feb. 13, two days after the 49ers’ 25-22 overtime loss to Kansas City in Super Bowl LVIII, the four-time team captain agreed when asked if he will require a more “involved” meniscus procedure. He said his recovery could extend into at least late July when the 49ers report for training camp.
Based on Armstead’s outlook, it’s very likely he required a meniscus repair, which, for those with meniscus injuries, is a less common procedure that results in a longer recovery.
https://www.msn.com/en-us/sports/nfl/post-surgery-recovery-outlook-for-49ers-arik-armstead-as-detailed-by-a-surgeon/ar-BB1jbNs4
The recovery timeline for a repair is often 4-6 months.