I disagree that managers should always have a predefined tactic/formation/philosophy in mind. In my opinion, there's more than one ways to go about creating a tactic. The two main ones being: creating a tactic based on player suitability (as far as roles and positions) or having a predefined tactical concept in mind when deciding how to play. An example of the latter approach would be Mourinho's tactical style (4-2-3-1 with holding/controlling midfielders and if possible inside forwards) - he has been utilizing this system at most clubs he's managed. Other managers are known to create more flexible tactics based on player material or, in some cases, adapting to your opponent. Ronald Koeman comes to mind. Another example would be van Gaal, who deviated from the classic 4-3-3 "Dutch school of football" philosophy by changing his formation to a 5-3-2 system (greatly improving his chances of winning) during the 2014 World Cup in Brazil.
Obviously there also exists a large middle ground between these two approaches. One can have a predetermined system in mind, but tweak it extensively to specific player suitability. Guardiola has done so with the Bayern squad: while he generally employs a characteristic formation & philosophy, he has changed the formation (4-3-3 to 4-1-4-1) to bring the best out of his players.
Having a high quality squad means that implementing a predefined philosophy will be easier compared to a lower quality squad, as players will generally be more intelligent and well-rounded resulting in higher adaptability. This holds true for both real life football and FM. It's up to you (as the manager) to decide to which extent you will tweak the many fantastic tactics shared on this board, in relation to your squad. That is also what makes this year's edition of FM so much fun, since there hasn't really been an unbeatable plug&play tactic around, meaning we are forced to invest in tactical optimization aka using your brain.
