Marcus Rashford has shared the advice he has given to Manchester United youngster Kobbie Mainoo. There’s not been much to be happy about for Manchester United in this underwhelming campaign. Erik ten Hag has seen his team ravaged by injuries, finish bottom of their Champions League group and slip to 11 Premier League defeats already. But if there’s one reason to be cheerful and optimistic it has been the rise of Kobbie Mainoo in the first team. The midfielder is still just 18 years old but has thrived when given an opportunity in the first team and is now one of the first names in the starting…