I really don't care. They could wear casual if they want. I think only thing is, no shorts and no "small t-shirts" (like tank tops or sleeveless), got to act a little professional
I mean FWIW, in the FIFA World Cup it felt like at least 1/4 of the managers wore casual clothing on the sidelines most games. Hell, Senegal's manager (forgot his name) was wearing a sweatshirt and sweatpants for most of the games
USA's coach, Berhalter, looked dressed like he was going to go shopping some days haha