WJ is dead on - that's not effective tactical thinking. Rocheyb sums it up nicely in another thread,
here.
Focusing entirely on the players best role is basically telling them go out there and do whatever they do best. That's it. Not trying to create any sort of cohesive tactical vision, unified approach, or anything. "Oh, you're a fast winger, better run down the line and knock in a cross. Don't have anyone in the box who can do anything with it? Ah well, keep doing what you're good at."
Don't get too caught up on players best roles. Not to say they are irrelevant, but they are recommendations and relative. Its more about fitting the player into your overall tactical approach considering what you need them to do and what they are capable of doing. There are reasonable considerations - don't ask your converted center back to be a Complete Wing Back and then wonder why they aren't playing like Dani Alves in his pomp. An example from my current save.... I use a 4-1-4-1 with a Complete Forward. Other forward roles could work but I like the diversity that role provides - it fits the approach. But I don't have any players at the current club that "can play" as a Complete Forward. I have 3 senior team strikes and one youth, and they are all quite incomplete. The best of them is a big forward who isn't terrible, but he's limited by the standard of the squad. On FM16, he would start in the reserves, and he was 25 years old (he's been improved a smidge on FM17. If I listened to the in-game roles he can play, I wouldn't use CF. Yet he has scored 5 in 5 and is tops in the league. He can't do everything a more rounded and skilled forward would in that role but he does the stuff I need well enough.