CHICAGO -- Pittsburgh Pirates rookie outfielder Esmerlyn Valdez left Friday's 12-2 loss to the Cubs after injuring his left hamstring while running to first base on a grounder in the sixth inning.
Valdez grabbed the back of his left leg and limped off the field with his arms draped around the shoulders of training staff after shortstop Nico Hoerner threw him out.
The Pirates said Valdez had left hamstring discomfort and was receiving treatment but gave no further details on the severity of the condition.
He was replaced in right field by Billy Cook, who misjudged Seiya Suzuki's fly ball on the first play in the bottom half of the inning and allowed it to fall for a triple.
Valdez entered batting .235 with 17 homers and 47 RBIs in 71 games. The 22-year-old made his major league debut on May 22 at Toronto.
The Pirates on Thursday placed All-Star right-hander Braxton Ashcraft (14-6), one of the majors' best breakout pitchers this season, on the 15-day injured list with arm discomfort.
Pittsburgh also activated infielder/outfielder Ryan O'Hearn from the 10-day injured list on Friday after he recovered from a strained left quad suffered on July 31.
The Associated Press contributed to this report.