From Darren:
Help? Is the block the “fullback”, or are a lot of “sweepers” needed to pick up the snow that drops from the players? I’m just really confused.
Plus the block is on the sidelines, not playing, it seems.
Commenters seemed equally baffled, including “This would probably be funny if I knew anything about soccer” and “That’s pretty ancient football… That was football of the seventies, with a Libero like Franz Beckenbauer. Or sixties, with an Ausputzer.”
Those don’t help.
Comics I Don't Understand 
It’s a goalie.
I have a feeling Gary Larson’s version would be funny.
The three players are being assigned positions, and so are entering the field before the game begins. A fullback is a defender playing a wide position. One of the three is clearly better at playing a wide position…
Yeah, I think it would have been funnier if the coach were pausing with “..now who should go in as goalie..?”