PHOENIX — On a day of few positives for the Mets, a displaced starting pitcher finally took a step forward. Sean Manaea pitched a scoreless eighth inning with two strikeouts in the Mets’ 5-1 loss to the Diamondbacks at Chase Field. The result was the lefty’s best in several weeks, but the most encouraging sign might have been his velocity. Manaea, whose fastball has lagged all season, averaged 91.3 mph with his four-seamer. He was averaging 89.7 mph with that pitch when the day began. In his relief appearance in Colorado on Wednesday he recorded only one out in the ninth, allowing three hits and plunking a batter. IMAGN IMAGES via Reuters Connect That letdown forced manager Carlos Mendoza to use Devin Williams for the final two outs with the bases loaded and the Mets ahead by five runs. “[Manaea] has been working really hard, you have to give him credit,” Mendoza said.…