Amad Diallo scored a stunning late hat-trick as Manchester United turned what seemed certain to be a humiliating defeat into a thrilling victory over Southampton at Old Trafford. United appeared to be heading for a fourth consecutive top-flight home defeat for the first time since 1930 as they laboured badly trying to overcome a Southampton side who had won just once all season and suffered a 5-0 home loss to Brentford on their last league outing.