If you want to boo, go right ahead and boo. It’s not something I would do regardless of how much an athlete mainly because I’m rather sure the player in question would not hear me individually or care in the slightest what some random person in the crowd whose never played a second of sports more than high school sports thinks about him. But I’m not going to get in your way of joining in with the rest of the butthurt fans and letting an athlete know you in fact do not like that they left your team and scorned you. But if you are going to say boo loudly at a player you once loved and now don’t like them because they decided to win somewhere else or get paid more money DO NOT do it while you wear that player’s jersey.
You shouldn’t have to be told not to do this. It be like confronting your ex girlfriend out at the bar wearing that hoodie she got you. You’re pretty much saying you’re not over them and they still own tons of real estate in your head and at the drop of a hat go back with them if they only gave you a look. At least be one of those weird fans that take time out of their lives and burn the jersey in a sort of sacrificial ritual. Doing it that way at least looks like you’re looking to move on from the player. Still ridiculous, but it’s better than wearing a jersey of a guy to the game, surrounded by other people in public and expressing your displeasure with that player every time. This is the ultimate oxymoron and could possibly be the ultimate fan fopaux you can do.