ARLINGTON — The Rangers have decided that the best classroom for Michael Choice’s continuing development is Triple A.At least for now.In a move speculated about for weeks, Choice, mired in a seasonlong slump, was sent down to Round Rock on Monday. Outfielder Jake Smolinski and right-handed reliever Roman Mendez were called up.In a related move, the Rangers designated Donnie Murphy for assignment.“We still have a lot of faith in [Choice],” manager Ron Washington said. “We don’t need his struggles to become mental. We want him to go down and work on the adjustments he needs to make and hope to see him soon.”Choice, in his first full big league season, was hitting .177 with eight home runs and 28 RBIs in 69 games. His troubles mounted in the last few weeks. The Mansfield Timberview and UT Arlington product was 5 for 66 in his last full 19 games, including going hitless in his last 14 at-bats before a pinch-hit single Sunday.“Obviously, he’s struggled,” said general manager Jon Daniels, who orchestrated the off-season trade that brought Choice from Oakland for Craig Gentry. “He’s been trying to make adjustments on the fly, which is a tough thing to do.”Daniels, while acknowledging it’s not “apples to apples,” compared Choice’s struggles to those of Nelson Cruz, who was acquired in a trade from Milwaukee in 2006.Cruz needed the better part of three seasons and a couple of stints at Triple A before becoming the player who has 184 career home runs.“We think it will be a quicker turnaround,” Daniels said. “Quite a few players don’t stick [at first] and contribute from Day 1.”Washington said Smolinski, 25, a second-round pick of the Washington Nationals in 2007, will be used in much the same way Choice was. Smolinski will spell all three outfielders.Smolinski, who had hit safely in seven of eight games with Round Rock since being promoted from Double A, struck out in the seventh inning Monday as a pinch hitter in his major league debut.Mendez’s term with the big leagues is expected to be brief. He was called up to fortify a bullpen that was, in Washington’s words, used “pretty hard” in Baltimore and New York.The Rangers have 10 days to trade, release or send Murphy to the minors.Another injuryIt was also no real surprise to learn the Rangers had been hit with another injury. This time, it’s pitcher Nick Martinez, who will miss Tuesday’s start with what was described as soreness in the rib cage.Martinez began feeling the soreness after taking batting practice last week in preparation for the interleague series with the Mets. It’s something he’s dealt with before, but this time it “lingered.”Right-hander Phil Irwin, a one-time Pirates farm hand claimed off waivers by the Rangers, will start instead. Irwin, 27, was 4-1 with a 1.74 ERA in six starts with Triple A Round Rock.The 27-year-old, who has been beset by elbow injuries, has made one big league start.Yu the All-StarA couple of weeks ago, Yu Darvish said he planned merely to do some sightseeing if selected to the AL All-Star team.Now that he won’t pitch Sunday, he seems to be all good with it.On Monday, he called any chance to pitch in the game “a once-in-a-lifetime experience.”Maddux hosts heroesThe Rangers and Hawaiian Falls water park are hosting families who’ve lost a family member while serving in the military at 11:30 a.m. Tuesday at Hawaiian Falls in Mansfield (490 Heritage Parkway South).Rangers pitching coach Mike Maddux is scheduled to greet families. Hawaiian Falls and the Rangers have teamed with Snowball Express, a nonprofit that, according to a press release, “provides hope and opportunities to create new memories for the children of military fallen heroes who died while on active duty since 9/11.”Briefly• Instead of designated hitter, heralded prospect Joey Gallo played first base at Double A Frisco on Monday to give him time off from the more demanding third base. It had nothing to do with any scenario with the big league club, Jon Daniels said.• Derek Holland is scheduled to throw a bullpen session Tuesday.