I think
'playing for the shirt' meant giving full commitment and being totally professional giving it your all always, being discipline and try to always give best performances. Keyword is always try to do just that. Whether the player really love the club or not doesn't matter. There are players who doesn't have affection about the club and still give it their all. Obviously having affection to the club helps with the motivation.
It's not just footballers, we can see that kind of pattern at any kind of jobs -- there are those who are fully committed irrespective of the environment/boss (former), and those whose performances are highly depended on the environment and especially the boss/manager (latter). It's normal.
Manager is the biggest factor.
A good balance of respectfulness, responsibility, strictness and leniency is ideal for the whole team to perform, both but especially to the latter. Having that annoying boss or rubbish shitty irresponsible manager have huge impact though, the former will try but still have their performances dip, while the latter will either don't care or just taking it easy cutting corners et all. Getting these kind of boss is a dream, but realistically, no boss is perfect, they shall always have their bad days and make mistakes. Getting a decent one that is almost like this ideal good manager is good enough. Being positive at least is a good start.
Either a decent effective manager or a positive manager.
Come on, let's admit it, when your boss is shit or the working environment is depressing, you don't feel the motivation to perform your work well. It's just not comfortable, discomforting feeling. Depressing even. Difficult to perform nor keeping your professionalism/discipline with this one. We may even escape from work, working lazily, and to some of us, gets emotional breakdown/outbursts. It's like a test of patience.
Reason you stay is more to get the necessary financial rewards to continue living. Let's say you really love your work, it still feels hard to get up early in the morning going to work letting your boss do shitty things on you, or even cannot sleep well at night because the next day you'll be facing the endless cycle of annoying boss having a constant go at you. Often than not, you'll be blame actively, always, or even told to do this and that, even the ones not within your job description, like a servant/slave. It's better if this boss make an efficient working system, but in the case he/she is so clueless about it resulting in a shit system, yeap, the same case, you will be blamed, the boss won't take any responsibility, why because the boss thinks his/her system is perfect. In addition, when your boss is having a bad that, for sure you'll be on the receiving end.
It's like bullying, letting ourselves to become the victim of bully for money. That's the
active bossy boss.
The more
passive bossy boss, is the lazy type who will skip working hours while throwing the team he/she's supposed to be leading giving them all the works, while this shitty boss just chill around probably holidays somewhere or going to fancy places/restaurants on a date, while we're here working in this confusing system of working environment. Then, when something bad happened, we will always be the ones to be blamed. Like where were you damnit. More often that not, this boss make a shitty system of working environment, confusing as hell, which affects our performances even further.
Morale Damages.
At the end of the day, it's still to get money, life sustenance.
Why stay at a depressing job, or working for a shit boss. The only thing we can do is if my boss is having a go at us, we'll reply in kind with a shit performances of work. I'm not paid enough for this shit to be blame, abused, used as a slave tool or punching bag, etc. If my boss is not performing giving it his/her all, then why should we. If the boss is a clueless idiot, we're on a downward spiral, sinking ship. The nice ones among us, the types that always try to give it their all, will still have problems at work, and at times will always have emotional burst, or emotional breakdown anyway, while the other types will cut corners.
Doesn't matter if you love the company, if your boss is shit, your performances dip.
In football, doesn't matter if you love the club, if your manager/coach is shit, same thing, performances will dip.
Players are still human beings.