By Simon Mullock Kobbie Mainoo still has work to do to get to the Euros – but the teenage midfielder is already indispensable for Manchester United. Mainoo staked his claim for a place in Gareth Southgate’s squad for Germany last week with a clever cameo against Brazil and a performance that belied his tender years on his first England start against Belgium. It didn’t take long for the debate to start raging about whether Southgate should now put his trust in the 18-year-old when the real stuff starts in the summer. Ten Hag is too long in the tooth to offer the England manager any advice – not…