He isn't a failure in big matches. He scored plenty of goals for United in big matches. We did win 3 league titles on the trot and a CL final with him in the team. Not to mention his goal in the CL final that got United the lead against Chelsea.
But the thing with him always has been, he can't be the creative force in your team - if you expect him to do that, he and the team will suffer. As for not playing him...yeah ok, leave out the most expensive footballer in the world, and 2nd best player.
Ronaldo is to blame - his best season with United wasn't the season he scored 40 odd goals, but the season before that. Where he still played as a winger/wide player, and created as many goals as he scored.
But something changed, and he decided, all he wanted to do was be a goalscorer, and well he has become a deadly goalscorer, but his all round game has suffered as a result.