This one is being reported as a stress fracture of his ankle that was just starting to develop.
Since they caught it early, it's very much treatable, and he should be back to full speed in about a month.
If I recall correctly, Tony Brooks had a similar kind of stress fracture during the the late part of the 1990 season. He tried playing through it during the Orange Bowl, and turned in a very underwhelming performance, even fumbling away once that made the difference in the game.
Between that and his academics, he sat out during the spring of 1991, and came back stronger than ever, putting together a very fine season in 1991.
Link: https://notredame.rivals.com/news/additional-ankle-surgery-sidelines-notre-dame-qb-riley-leonard