That would be the case if writing for a more general audience... but in a publication whose audience is, say, digital marketing executives, "ROI" would be perfectly fine. Of course, depending on the editorial guidelines that publication follows, it might not be allowed because they don't like acronyms, but not because it needed a definition.

With sports writing, you can't expect definitions for common sports jargon. In F1 racing you have to assume that readers know what "pole position" is, in football (US: soccer) you assume readers understand "offside", and so on. If you were to explain every piece of sporting terminology in every article, it would drive readers crazy.

As to acronyms, while it makes it even more confusing for people who don't understand the sport, the vast majority of the readers are people who do understand it. And for those who don't, it often wouldn't make a difference. If I wasn't a baseball fan, "earned run average" wouldn't make any more sense to me than "ERA".

The biggest difference between American sports and European sports (I'm not familiar with MLS, so I'm not sure if this difference lies with the sport or where it's being played, MLS perhaps fits with European football or perhaps with sports such as baseball) is that American sports use way more terminology and statistics, not just from journalists, but talked about by fans, shown in stadiums, etc. If you watch a US baseball game, and a cricket game (not that cricket is all European, but it is non-US), you'll see that difference hugely.

And that's actually something that fascinates me - I'm English, so such a big focus on stats wasn't a part of sport for me growing up, but once I got into NFL/MLB, I really love it. But, it still seems like something that most sports fans wouldnt care about, to me. Even though I know that they clearly do.