Its the "I didn't play him because he had an orange injury" counting as "Not playing him" for happiness purposes that always bugged me the most.
magacid, at this point I'd probably look at the praise bit. Combined with your observation that he had a concern about "How you spoke to players on match day", you might try giving him specific Team Talks, instead of just giving him the generic one you're giving the rest of the team. I'd recommend things like "go from strength to strength", "you can make the difference here", specific-player-"Pleased" or "Delighted" team talks, etc.
I
would keep a close eye on him - the other problem is that you've bought a high-Controversy prima donna who expects you to cater to his every whim and is going to blow up when things don't go so well.
If team talks still feel like a minefield to you, you might want to try reading
Communication and Psychological Warfare over in the Tactics & Training Tips forum - unless you've got that LLaMa-esque "I don't want any help, dagnabbit!" ethos.