Drew Sharp at the Detroit Free Press puts his finger on the problem when commenting on the Lions lack of discipline:
[Head coach Jim] Schwartz didn’t take the off-field nonsense seriously last year. It reflected his head-coaching inexperience. Commitment is a 24/7 proposition in the NFL. It isn’t a light switch you can flick off when you’re not at minicamp or OTAs and then flip it back on when reunited with your teammates and coaching staff.
It always bugs me when I’ll write something about an off-field problem with a player and fans will come back at me with “I don’t care about what kind of citizen they are. I just want a good player on the field”. The problem is that you are talking about players as whole people. There is no “here’s how I am off-field” and “here’s how I am on the field”. Its always just “here’s how I am”, period.
Bottom line, a lack of discipline off-field will almost always show up on the field in some way. It won’t always hurt you as badly as it has the Lions. But it will hurt you.