Washington Spirit captain Ali Krieger, a probable U.S. national team starter at the Women's World Cup this summer, was taken off the field on a stretcher Friday night in Houston after suffering a head injury. Krieger, a Northern Virginia native and one of American soccer's most experienced defenders, was treated on the field for about seven minutes and transported to a hospital for testing. A scan and X-ray came back clear, according to her father, Ken, a longtime youth coach in the D.C. area. "The team trainer will keep a close eye on her," said the elder Krieger, who believes she suffered a concussion. He was awaiting an official diagnosis. She was expected to leave the hospital later Friday night and return to the team hotel. Concussion protocols set by the National Women's Soccer League and U.S. Soccer Federation were being followed, the Spirit said in a written statement. The initial diagnosis was a "mild concussion," the team added. "She is doing OK," coach Mark Parsons said. "It wasn't pretty. She was in a bad way. We were all a bit worried." Krieger, 30, was injured in the 17th minute of the National Women's Soccer League's season opener, a 2-0 victory by the the Houston Dash over the Spirit at BBVA Compass Stadium. On a Houston cross into the penalty area, Krieger rose for a header. Houston's Jessica McDonald also attempted to head the ball and appeared to make contact with the side of Krieger's head. Krieger lay flat on her stomach as players waved for assistance. The medical staff placed a brace on her neck and used a spine board before fastening her onto a stretcher. Spirit goalkeeper Ashlyn Harris, a U.S. teammate, was at her side for almost the entire time. As she was wheeled off the field, Krieger waved to the applauding crowd of 6,012 and spoke with Harris. Krieger, The Washington Post's All-Met player of the year at Prince William County's Forest Park in spring 2003, suffered a concussion in August 2013 while playing for the Spirit and was sidelined for a few weeks. The latest injury comes less than two months before the World Cup in Canada. Krieger, a right back, started all six matches at the 2011 tournament in Germany and converted the winning penalty kick during a quarterfinal tiebreaker against Brazil. Last weekend, she played 83 minutes in a 4-0 friendly victory over New Zealand in St. Louis. The Spirit's home opener is next Saturday night against defending champion Kansas City at Maryland SoccerPlex. Krieger, Harris and Crystal Dunn are tentatively scheduled to rejoin the national team in early May for the start of World Cup training camp. The Americans, ranked No. 2 in the world behind Germany, will play tuneup matches at home against Ireland, Mexico and South Korea before opening Group D play against Australia on June 8 in Winnipeg. Carli Lloyd scored in first-half stoppage time for Houston, and Kealia Ohai added a second-half goal.