Kobbie Mainoo had to wait until November’s 3-0 win at Everton to make his senior Manchester United debut. 12 months on, could another Carrington kid be about to announce himself on the first-team stage? Erik ten Hag, speaking after the 3-2 pre-season friendly victory over Real Betis in midweek, offered a glimpse into his plans for teenage left-back Harry Amass. The rampaging 17-year-old, Ten Hag tells the Manchester Evening News, is not yet at the level of Mainoo or Alejandro Garnacho when they exploded onto the scene. “But if we train him well, if we coach him well, then in three months (come…