ARLINGTON, Texas (AP) -- The Texas Rangers want struggling rookie outfielder Michael Choice to work on his game in the minor leagues while they replace yet another pitcher in their injury-riddled rotation.
The Rangers demoted Choice to Triple-A Round Rock on Monday with his average at .177 after just six hits in the past month.
''He needed to go get himself together,'' manager Ron Washington said. ''We have a lot of faith in him. It's just that you don't need the struggles to become mental.''
Infielder Donnie Murphy was designated for assignment, allowing Texas to bring up right-handed reliever Roman Mendez and outfielder Jake Smolinski from Round Rock. Both could make their major league debut in the series opener against Houston.
Rookie right-hander Nick Martinez was scratched from Tuesday's start because of a pulled muscle in his rib cage, apparently sustained while batting during an interleague series with the New York Mets.
He will be replaced by right-hander Phil Irwin, who made his only big league appearance in a start for Pittsburgh last year. Irwin is 4-1 with a 1.74 ERA in six starts for Round Rock.
It's likely that Martinez will be placed on the disabled list but only miss two starts because of the All-Star break.
''He wants to start tomorrow, but just trying to be smart,'' general manager Jon Daniels said. ''Irwin's been throwing the ball well at Triple-A. Looking forward to seeing what he's got.''
Irwin will be the 12th starter used by Texas, one shy of Colorado's major league high.
The Rangers acquired Choice in an offseason trade that sent utility outfielder Craig Gentry to Oakland. He was a September call-up for the A's after spending the year with Triple-A Sacramento and was good enough in spring training this year to make Texas' opening day roster.
''Very few guys come up and stick and contribute from Day One,'' Daniels said. ''That was our hope. There were times we thought that might be the case.''
The 25-year-old Smolinski was a second-round pick by Washington in 2007 and spent his first seven professional seasons in the minor leagues. He spent most of this season with Double-A Frisco, where he had 10 home runs in 72 games, before a recent promotion.
The 23-year-old Mendez missed most of 2013 after surgery for a stress fracture in his right elbow. He was 0-1 with a 4.02 ERA in Round Rock.
The Rangers have 10 days to trade, release or outright Murphy to the minor leagues. He was hitting .196.