The front three of Swanson, National Women's Soccer League (NWSL) MVP Sophia Smith (22) and veteran Alex Morgan (33) were expected to produce fireworks in Australia and New Zealand but the defending champions' plans were thrown into disarray when Swanson suffered a torn patella tendon on Saturday.
After Tuesday's match in St. Louis, Missouri, the final game before head coach Vlatko Andonovski must name his 23-player roster, it was clear she would be tough to replace.
"The team was built - or was going to build - around Mal and Soph and their attacking power. Now with Mal not being there, we're going to have to make a decision. What are we going to go for?" Andonovski told reporters.
"It's hard for me to answer this question right at this moment."
Alyssa Thompson (18) started up front in place of Swanson but it was Alana Cook's (26) goal late in the first half that gave the US the win, the defender's strike from distance marking her first international goal and coming on her 26th birthday.
"Mal's shoes are so big," captain Becky Sauerbrunn (37) said. "If there are any attackers right now playing in the NWSL, playing overseas, now is your time to shine."
Andonovski told reporters he gave as many players as possible the chance to stake a claim before he names his World Cup squad.
"It was hard to evaluate the team and team performance in a setting like that when there are a lot of changes, players that haven't played together over a year, year and a half, and players that haven't even played at all, or (for) a year and a half," he said.
The U.S. will play Wales on July 9 in San Jose, California before they travel to New Zealand for their bid for an unprecedented third consecutive title.