I think some confuse Cooper having a low-key personality with him not caring. That's never been the case. Cooper's main issue has always been health. He has some chronic lower-body issues that slow him down. Without those, I think he'd be mentioned with the best in the game because of his route-running acumen. It's hard to get in and out of your breaks as quickly or run at top speed when you've got plantar fasciitis or muscle pulls. Cooper hardly misses games despite his injuries, so some don't even notice when he's playing hurt. Most of the time, if he's not playing well, he's playing hurt.
He's one of those players, like Sean Lee, that you would like to see what their careers would look like without all of the injuries. Unfortunately, that's not how their bodies are made, so you can't expect them to be something their bodies won't let them be.