WASHINGTON — Bryce Harper appeared to injure his left knee when he slipped on a wet first base, a potentially devastating blow to the NL East-leading Washington Nationals.

While running out a ground ball to the right side of the infield in the first inning Saturday night against the San Francisco Giants, Harper avoided contact with first baseman Ryder Jones and his left foot slipped on the base. His leg buckled , he went airborne and fell to the ground as his helmet went flying.

Harper clutched at his left knee after going down and was attended to by trainers. The star outfielder put no weight on his left leg as he was helped off the field by hitting coach Rick Schu and head athletic trainer Paul Lessard.

The game began just after 10 p.m. following a 3-hour rain delay. It was raining steadily when Harper's injury happened.

Major League Baseball was in control of weather decisions because it's the Giants' final trip to Washington this season. The series opener Friday was postponed after a lengthy rain delay to be a part of a day-night doubleheader Sunday, adding to baseball's incentive of getting the game in Saturday.

But the game is unlikely to have an impact on the playoff race as the Nationals lead the NL East by 14 games and the Giants are out of contention. At the most, it could have an effect on home-field advantage in October.

Even though first baseman Ryan Zimmerman is having an All-Star season, Washington's title hopes hinge largely on Harper's health. The 2015 MVP is having another spectacular season, hitting .327 with 29 home runs and 87 RBIs this season and played in 105 of the Nationals' first 113 games.

Harper didn't last long in his 106th game and was replaced by Andrew Stevenson, who took over in centre field with Brian Goodwin moving to right. Despite many fans waiting out the delay, the stadium went eerily silent in the minutes after Harper's injury.

Because of a bevy of injuries, the Nationals have used 12 outfielders this season. They have been without Adam Eaton since late April because of a torn ACL in his left knee that ended his season, Jayson Werth since early June because of a bruised left foot and Michael Taylor since July with a right oblique strain.

"There aren't many organizations that have 12 outfielders that they can really rely on and count on," manager Dusty Baker said Friday. "Boy, that is a lot, especially when you only play three at a time. I'm just glad we have them."

The Nationals are running away with the division despite those injuries and more to starters Stephen Strasburg and Joe Ross, relievers Shawn Kelley and Koda Glover and shortstop Trea Turner, but a long-term injury to Harper would be far more trouble to Washington's post-season hopes.

Harper is under contract through next season and can be a free agent in the winter of 2018.

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