A mullato, my libido
An team which isn't cohesive is not a team, its a group of individuals.
If you watch the game on Full or Comprehensive, as you ought to, you will see passes that don't go straight to feet, or first touches that are slightly delayed or wayward. At its worst, a lack of team cohesion will see your pass completion ratio plummet, runs go unnoticed and movement be completely senseless, all of which will undermine the success of your tactic (and MAKE YOU THINK THE ME IS BROKEN WHEN ITS NOT!!!).
The most obvious form of team spirit is how well the team is blended, and this depends on a combination of things:
1. Time spent together
This is the most obvious one and it is probably one of the reasons that in real life Barcelona have dominated football for the last few years. Most of their squad has come from La Masia. If players spend so many years with one another, they' re much more likely to know their role in the team and how to complement each other, and this is paramount to playing good football.
2. Languages spoken
If a player speaks a different language to the rest of the team, it severely damages his ability to play well and link up well; the impact is lessened if he is a less creative role where synchronisation is less important, but it is there nonetheless. It takes over a year for a player to fully learn a language, but less if you have a player of the same nationality who can speak both languages. Remember that South American and African countries tend to be former colonies and players from these countries will speak one of the major European languages, which is one less thing to worry about (Brazilians speak Portuguese, Argentinians Spanish, Ivory Coast French etc).
3. Personalities
Again all of the Barcelona squad have a broadly similar personality. There's no-one really in their squad, or in any successful team, who has a personality out of kilter with the team ethos (see also Man Utd till Moyes came in and did his best to destroy us or the **** under Wenger).
If you are patient you can change personalities and breed this ethos into players from your youth team by using stuff like tutoring etc.
4. Favoured personnel
The highest level of blending a team can achieve on FM is willing to die for each other and this should be the ultimate team building aim for managers. You want your players like one another as this will prevent disagreements and help them to link up more often and with better quality. There are obvious team cohesion/blending/spirit advantages to having players who like and appreciate one another as friends.
How to encourage blending:
1. Watch matches closely, look at player stats etc and see who is not blended.
2. Only play one or two unblended/new players at a time to slowly blend them into the squad and allow them to feel their way around your style. This will reduce their negative impact and allow you to grind out some results in the early stages of a season. This is slow but means you' ll be able to keep the board's and fans' confidence and get some decent form going. See previous post about avoiding second season syndrome for more on this.
3. Expect unblended/ new players to suffer in form till they blend. Dont be quickly disappointed or lose faith. Talk to them and utilise the option "I know you havent been scoring recently but I have faith you ll get through it" or similar. Throw them straight in the deep end and let them fight through for form. But not more than 1/2 per starting XI. This will be the quickest way of blending them into the squad, as they will get used to playing with one another, your existing players, and your tactics, but you will most probably get some badresults unless you try to do as per my post on second season syndrome. In my career as an FMer I very rarely if ever experience second season syndrome following exactly the advice Im giving you.
At the same time be the boss!
1. Dont let upset players call the shots.
2. Dont allow players to leave for any other reason than because you dont need/want them any more.
3. Do not give in to transfer requests.
Remember that in a team with most players on excellent morale you can afford to have 1/2 with bad morale because they ve fallen out with you. Keep playing the rebels and dont budge. Sooner or later they will kiss and make up (and while upset they'll keep playing for the team despite themselves - I lost count of the times players on red morale played well for me/scored crucial goals etc).