LOS ANGELES — The Washington Nationals scratched unbeaten right-hander Stephen Strasburg with an upper back strain, spoiling his scheduled showdown with Clayton Kershaw.

The NL East leaders announced the move about 80 minutes before the first pitch against the Los Angeles Dodgers on Monday night.

Yusmeiro Petit got the start in place of Strasburg (10-0), who was due to face Kershaw (10-1) for the first time in their major league careers.

CUBS: Chicago placed center fielder Dexter Fowler on the 15-day disabled list with a right hamstring injury.

Fowler got hurt while running out a grounder to third in the first inning of Saturday night’s 4-3 victory over Pittsburgh. Fowler, who re-signed with Chicago in late February, is batting .290 in 64 games this year and was leading the NL outfield voting for the All-Star Game.

TIGERS: Detroit purchased the contract of right-handed pitcher Dustin Molleken from Triple-A Toledo. To make room for him on the 25-man and 40-man rosters, the Tigers designated infielder Casey McGehee for assignment.

The Tigers also transferred right-handed pitcher Drew VerHagen to the 60-day disabled list with right shoulder thoracic outlet syndrome.

Molleken, 31, has never pitched in the major leagues. He has pitched 13 pro seasons overall, including two in Japan for Nippon Ham.

He’s 1-3 with a 3.76 ERA and a 1.44 WHIP in 23 games for the Mud Hens.

RANGERS: Left-handed reliever Neal Cotts signed a Triple-A contract with Texas.

Cotts was 10-12 with a 2.84 ERA in 131 games for the Rangers from 2013-14. Cotts, 36, had a 3.41 ERA in 68 games combined last season with Milwaukee and Minnesota.

RAYS: Outfielder Steven Souza Jr., who was placed on the 15-day disabled list last week, received an injection in his sore left hip.

• Right-hander Cody Anderson, on the disabled list at Triple-A Columbus with a sore elbow, is expected to miss at least a month.

PHILLIES: The Phillies signed No. 1 overall draft pick Mickey Moniak.

A high school outfielder from California, Moniak hit .476 with 12 triples, seven home runs and 46 RBI in his senior year at La Costa Canyon High School.

Moniak was the first prep outfielder selected No. 1 overall since Delmon Young went to Tampa Bay in 2003.