Football Manager 15 is a massively comprehensive game like its predecessors, but even the developers at Sports Interactive can't include everything. The title is only in beta right now for those who pre-ordered until it releases next month, but the community has already provided much of the real-world content not included in the game.
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The fans at SortitoutSI have provided missing player faces, replaced the generic club logos with real crests, and added in anything else the studio was not allowed or able to include.
There are download packs for each aspect of the game available, from kits to player faces, though they no doubt circumvent any license deals Sports Interactive couldn't or didn't sign. Fans in many game communities rally around this type of mod or add-on, but the speed SortitoutSI managed this with is impressive.
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It was likely possible because the same data and images were done for Football Manager 14, making it just a task of the team carrying the files over to the new release. Most kits do change from year-to-year, though, so that portion of the offerings must have been done in advance of this release (unless these designers are extraordinarily quick). These are not officially sanctioned add-ons, of course, but like any fan mod can add in that missing aspect of the game players care about most. The real faces and logos are certainly preferable to the blank silhouettes and generic crests--check them out for yourself.