Zack Wheeler’s promotion is a question of when, not if. The top prospect is expected to reinforce the shaky backend of the Mets starting rotation at some point this summer. But he still has a ways to go. The right-hander surrendered five runs over five innings for Triple-A Las Vegas Thursday. He struck out seven, but issued three walks and allowed six hits against Tacoma. The disastrous performance followed a six-walk effort over 4 1/3 innings on April 19. Mets manager Terry Collins spoke with members of the Las Vegas coaching staff today and the consensus was Wheeler’s control is the issue. “What worries me the most is that he’s not pounding the strike zone,” Collins said. “We’ve got to have some strikes out of him because his stuff is going to play. If he’s in the strike zone, you’d be surprised at the outs because his stuff is just that good.” Wheeler has a 5.79 ERA over 23 1/3 innings in five starts. He’s walked 15 to 28 strikeouts. “I spent 12 years in the Pacific Coast League,” Collins said. “I know how the numbers can be distorted so it’s not about the numbers. It’s about commanding the strike zone.”