Krieger did resume training last week with the Spirit, wearing a red shirt to remind her teammates to avoid making contact with her, but she said was still not ready for game action. She has since left the Spirit to join the national team as it prepares for the World Cup, where she is expected to be a starter when the tournament begins in June.

Still, even a month after her injury, Krieger knows she cannot be certain that there will be no setbacks. She has spoken with Twellman and other players who had to retire because of head injuries but acknowledged, “No one really knows how to deal with a concussion.”

She added: “The only way I can explain it is, it’s kind of like a snow globe you shake up. You have to wait until all the stuff falls and just rests. At the start, it’s a mess, and you have to wait. And right now, I’m feeling like the pieces are all settled and I can get back to training.”

Soccer and other sports continue to rely on injured athletes to convey their conditions to trainers and doctors, even as their competitive instincts push them to return the field. A stage like the World Cup can be tough to leave.

“If you took M.R.I.s of our bodies,” Krieger said, “I’m sure you’d find little fractures, things we don’t even realize because we’re conditioned to just push through it. We grow up like that. We figure out a way to just tape it up. We’re so used to it, and so competitive, that we don’t want to sit out.”

But even when players are willing and able to assess their health truthfully, the system is not perfect. Asked if he remembered whether a coach or a doctor ever told him to play when he should not have, Twellman said, “Of course I do.”

Still, he added, “I have to ask myself, ‘Was I completely honest with the trainers during the process, too?’ ”