Welbeck was injured for much of last season, but still had a standout performance against Real Madrid. The consensus for Kagawa's first year was that he wasn't especially good and still needed to find his feet, though he did put in a terrific performance against Norwich. Overall, neither had a great time, but Fergie must have had a reason for loving Welbeck and playing him whenever he was fit. SAF also persevered with Valencia fwiw.
Coming back to Kagawa, since he joined the club various excuses have been made for his lacklustre performances including his position/his teammates/the style of play/the manager. I have no vested interest in seeing him fail to adapt, but there has to come a point where you say that perhaps it isn't everybody else's fault and maybe it's just down to the player.