The Notre Dame secondary allowed 205 passing yards per game last season, which was good for 38th in the country.
However, with Harrison Smith and Robert Blanton no longer around, the Irish were already looking for replacements in the defensive backfield. Things got even worse in fall camp when projected starting cornerback Lo Wood suffered a season-ending injury (via the Chicago Sun-Times).
This secondary is now paper thin, which is not good when you have a defensive line that can't generate pressure consistently.
The Irish have found a solid cornerback in Bennett Jackson, but replacing Wood will likely be Jalen Brown, who did not see any playing time last year.
The good news is that the safety positions will be held down by upperclassmen Zeke Motta and Jamoris Slaughter, but there isn't much depth behind them.
With that being said, head coach Brian Kelly knows his players are going to have to learn on the fly (via The Washington Post):
There’s one thing that scares me the most and that’s when you have slow corners. We don’t have any slow corners. All those kids can run. Jalen can run. Josh can run. Now it’s about honing their skills. They can certainly athletically do the job. Now it’s a matter of how quickly they can pick up the technique and discipline, which is so important in playing that position.