It wasn't simply "not as good." It was catastrophically worse. That continues to be glossed over by many, as I noted earlier, like some kind of phenomenon adjusting to the acceptance that he's hurt. "15" is closer to what they were with Parsons. Probably right about 10th or so.
After the ACL injury, the Packers finished the regular season 0-4, allowed an NFL-worst 170.8 rushing yards/game, and recorded only 3 sacks, the fewest in the NFL over that span.
Post-Parsons: NFL-worst rushing defense, NFL-worst sack production, roughly bottom-of-the-league yardage performance, followed by 31 points allowed in the playoff loss. Yes, they lost some other guys too. But it shouldn't be a hot take that when you mortgage two firsts and pay a gajillion dollars to a superstar, your team is a LOT worse when he can't play. Just on the eye test alone, they were alarmingly worse the second he was out.
The one major thing the defense has going for it, is that it would be hard to be as bad as they were forcing turnovers for 2 years in a row. Just by sheer odds, they should do that more often this season.