Actually, as is his custom, CJF didn't give specific details.
From a Philadelphia Inquirer article from August:
"STATE COLLEGE, Pa. – Penn State’s Adisa Isaac, who was expected to be a strong candidate to take over one of the team’s two vacant starting defensive end spots, will most likely miss the entire 2021 season with an undisclosed injury, head coach James Franklin said Saturday.
Isaac, a 6-foot-4, 248-pound junior from Brooklyn, N.Y., played in all nine games for the Nittany Lions last season and was in on 13 tackles, including 1.5 sacks. He had 27 tackles, including three sacks, 4.5 tackles for loss and one forced fumble, in 20 career contests.
Franklin said Isaac’s injury did not happen during football training. He initially said the player would “most likely not be available for the season,” but later expressed the thought that he would “be out for a significant amount of time.”
“He is doing unbelievably well, so you never know, the way medicine is now,” Franklin said. “Adisa has been phenomenal in terms of, you know, doing what he needs to do to get back as quickly as he possibly can. So, you never know. But he’ll be out for a significant amount of time.”"