The people that post here are distance runners so they don't really understand how hard it is to run a 5K fast for most people.
To run a fast 5K you need some really specific distance training that isn't needed in almost every other sport. There is a reason fast 5K athletes run 90+ miles/week; it's an aerobic sport that requires very specific muscle groups.
If this guy can run 18:xx off soccer training, he's probably capable of sub 15 with some dedicated training.
I always wondered how good of a runner Landon Donovan could have been. Dude was just super fit and ran so hard all of the time. I bet he could have been a world class runner...
FYI:
In soccer the majority of goals are scored in the last 15 mins of play. Soccer is in some ways an endurance sport. If you are fit, you can play better in the last 15 mins and not make as many mistakes as your opponents. It may look as if soccer is a speed sport and it is. But it is really a sport about who can be fastest after 80 mins of hard repeated sprint efforts.