Sports law generally isn't a big practice, even at huge firms. It's true that a contract is a contract, but realistically anything football related is probably going to one or two partners. It's very unlikely in my view that there would be a completely different set of people working on Messi's contract matter than those who represented Barcelona in contract matters previously. I'm not a lawyer but I've worked with firms before in the past which, then in a later deal, sought to represent the party on the other side of me and in each case they had to come to us prior and request a conflict waiver before doing so (which sometimes were granted, sometimes were not). Skipping that step is definitely grounds for dismissal, putting everything else aside.