straight to your inbox every day for freeMason Greenwood was substituted in the second-half against West Ham because Manchester United are managing his game-time.Greenwood, 19, has started in six of United's last seven matches but was replaced by Edinson Cavani in the 86th minute, much to the surprise of many observers who felt he had performed better than fellow forwards Marcus Rashford and Anthony Martial.Martial was starting for the first time in four games but failed to score while