After never before starting a game in the outfield, Lyle Overbay played right field in three straight to end the homestand. But the key element there is “home.” Yankee Stadium has one of the smallest and least complicated right fields in the majors, which is a fine testing ground for a novice. But the Yankees now go on a 10-game trip that opens at Safeco Field, which even with moved-in fences from last year, has a vast right field, before heading to Oakland, which has a humongous outfield. That will not stop Joe Girardi from deploying Overbay, but it will be done more judiciously than at home. At least until Overbay proves he can handle the position competently, which he has done in the three games. Which does not surprise Yankees third base coach Rob Thomson much. Thomson is also the Yankees’ outfield coach and was a key voice in the Sunday night meeting in which Girardi and the coaching staff persuaded a dubious Brian Cashman to try the experiment. In fact, Girardi said if both Brennan Boesch and Overbay were available to him for Monday’s game against Cleveland, the manager was going to start Overbay in the outfield. Which led to Boesch being demoted, particularly because Cashman wanted Overbay around as insurance for injury-returning Mark Teixeira and injury-prone Travis Hafner. Thomson said one mental exercise he undertakes all the time is imagining, if disaster strikes with a game in progress, who could move to certain positions. So he had been thinking about Overbay as at least an emergency outfielder for a while, in part because of what he called Overbay’s deceptive athleticism.