The M.L.S. season that opens Sunday will be the league’s 21st, a fitting number for a year that will feature the debut of the 21-year-old striker Jordan Morris.

Morris, who joined the Seattle Sounders in January after a brief flirtation with Germany’s Werder Bremen, was the most noteworthy pickup for M.L.S. in a budget-conscious off-season. A former Stanford all-American, he arrives with a salary-cap-friendly contract and the unenviable task of replacing Obafemi Martins, a highlight-reel regular who was lured away by the deeper pockets of China’s Super League.

Morris’s ability to forge an attacking connection with Clint Dempsey in Seattle will have major implications for both the Sounders’ title hopes and the United States national team’s 2018 World Cup qualifying campaign. But it is not the only story line to follow this season.

■ If M.L.S. made waves in 2015 for its splashy signings of a handful of multimillion-dollar players, this off-season was more about buying off the sale rack, and the Los Angeles Galaxy did most of the shopping. The Galaxy added three veteran Europeans — the former England defender Ashley Cole and the bone-crunching tacklers Nigel de Jong and Jelle Van Damme — to a team that already had Steven Gerrard, Giovani dos Santos and Robbie Keane in attack. If anyone can make it work, it is Coach Bruce Arena, but the early returns have been mixed: The Galaxy were thumped, 4-0, by Santos Laguna of Mexico last week on their way out of the Concacaf Champions League.