I would like to politely suggest reconsidering the book's Frontpage, assuming there's still time before it goes to print.
Scala die-hard programmers will buy the book no matter what (myself included).
But I think you can do MUCH better if you expect to catch the attention of the curious. You should try using an image that goes hand in hand with the adjectives you would use to describe Scala.
Scala is a big language with a lot of features and a lot of power. At the same time I would say it's an elegant language. Like most functional languages, it's a language that makes you think.
With all due respect, soccer players are not the brightest people around and they don't really use their minds that much to make a living. This may sound harsh but trust me, our subconscious is unforgiving and you're trying to sell books aren't you?
Just run a poll/survey, show your programmer friends several images for the book and see how many pick the soccer guy. It can't hurt, right?
Again, this is just a polite suggestion and I don't mean to offend anyone. I just had to say it like it is without embellishing it because that would defeat the purpose.