In Football Manager 2018, there are a number of new Player Roles that can be assigned to your players. These roles will allow you to have greater control over each of your players, specifically in each of the positions that you are looking to use them in. Even though some people might believe that certain roles are most important than others, the truth is that not one of the new Player Roles actually stands out above any other. Each has their positives, varying depending on what the role is and who you are looking to use in said role.
Before trying to implement a player into a specific role within your Tactic, it would be wise to first look to see if said player is naturally comfortable with the new role you are trying to assign to them. On top of this, it would also be a good idea to view a player's Attributes in order to see if they are a good fit for a specific role on your team. If they don't have the Mental and Physical Attributes it takes to have success at a certain role, you might want to put them elsewhere.
But should a player have all what you believe it takes for them to be successful at a certain role within your ball club, don't hesitate to place them at said role and being developing them as such. Who knows, you might even be able to help groom and mold one of your players into becoming the next Segundo Valente, Mezzala or Carrilero for your team. These are some of the better roles a player can accomplish at earning, and they could see said player, should they be able to develop accordingly, into a star for many years to come.