Bryce Harper injured his left knee when he slipped on a wet first base, a potentially devastating blow to the NL East-leading Washington Nationals. Manager Dusty Baker said Harper hyperextended his left knee and was scheduled to have an MRI on Sunday. ''We just ask for prayers that it's not serious,'' Baker said after his team's 3-1 victory over the San Francisco Giants that was overshadowed by the injury to one of baseball's top players.