I think Athletics, and especially sprinting could be a slightly different matter though. A 100m sprint lasts for 10 seconds and most people I know really only care about Athletics whenever it's the Olympics. When it's 10 seconds of fun every 4th year, it's hard to care all that much about it on a regular basis.
If we go back to my previous example, about the fencer and the chess player, we'll see that the fencer's popularity dropped pretty much immediately after the Olympics. Big chess tournaments are being played all the time though, which means that the media here in Norway constantly remind us of how he's winning it all, and his popularity hasn't dropped.
If Messi was American, and tearing it up La Liga every weekend, he'd get constant media exposure, unlike the sprinter, and this would probably lead to Americans taking an interest in him, and football by extension. At least I like to think so.
