Marvin Luna fished into his bag and eyed the cleats he had brought, just in case. The 28-year-old had not been planning to play soccer. His knee had been acting up again, and he had agreed to work in his family’s food truck, parked near this patch of green synthetic turf. But as former teammates patted him on the shoulder on their way to the field — that field — it was hard not to think of his father.