Even before the rebuilding Astros traded veteran shortstop Jed Lowrie and the organization’s MLB-low payroll took another hit, the team was staring at holes in several key areas. The outfield, the back end of the starting rotation and the bullpen were completely up for grabs heading into spring training, while third and first base weren’t officially locked down. The Lowrie deal strengthened the Astros at first. Houston sees real MLB potential in Chris Carter, one of three prospects acquired from the Oakland Athletics for Lowrie. Carter, 26, had 16 home runs and 39 RBIs with a .514 slugging percentage in 218 at-bats last season for the AL West champion A’s. “The reality is, if he comes into spring training and executes the way he’s potentially capable of executing, we’ll find a spot on the club for him,” Astros general manager Jeff Luhnow said Tuesday during a media luncheon at Minute Maid Park. But Houston’s potential buildup at one position instantly opened a glaring hole at another. Lowrie was expected to be the Astros’ infield leader and form a strong duo with All-Star second baseman Jose Altuve. Now, the Astros will head to spring training with Tyler Greene (266 games played) and Marwin Gonzalez (80) battling for the right to be their everyday shortstop. Opportunity knocking It is possible neither will win the job outright. Barring another trade or signing, the Astros may well have Greene and Gonzalez platoon all season while occasionally being spelled by Jake Elmore (30 career games). Shortstop has replaced outfield as the clearest symbol of the Astros’ inexperience. The front office and coaches are hoping the Greene-Gonzalez combo becomes the equivalent of one solid MLB player. “Whenever opportunity knocks, sometimes the real player really shows up,” first-year manager Bo Porter said. “So we’re hoping that one of those guys grabs the bull by the horns and takes charge of the shortstop position.”