Last year, at an address to introduce the 17th Major League Soccer season, commissioner Don Garber acknowledged that "the ghost" of the North American Soccer League still haunted the corridors of MLS. The boom-and-bust excesses of the earlier league stood as a cautionary tale to those who would follow, and set the tone for the prudent-to-a-fault cartel that makes up the single-entity MLS.

This year, Garber stuck to his theme of steady growth, at the start of a season that has seen MLS overtake NASL—which lasted from...