By Alek Arend The San Francisco 49ers have a concerning update on one of their best defensive players. Star defensive lineman Arik Armstead has suffered a knee injury and will miss some time. Niners head coach Kyle Shanahan announced this Friday that Armstead, 28, sprained his MCL during training camp this week. Although it’s not believed to be too serious, Armstead is going to miss several weeks. “It’s nothing we’re too concerned about but it will take him some time away,” Shanahan said, via Cam Inman of the San Jose Mercury News. “He thought it was just a bruise, because it didn’t bother him…