I'll give you a pretty good summar of Nani.
Nani and Anderson came to us, both had huge raw ability, lot's of potential and for two seasons Anderson worked his bum off, adapting his game to the premier league, doing the dirty work, tackling, chasing back, becoming a team player, almost keeping himself and his own ability on a leash, sacrificing his own personal glory for the team. This season however, Anderson has started to bring his own brand of football to his game, so now he's perfected and tightened his game up, he's able to free himself up more, he attacks more, dribbles, shoots, looks for killer passes etc.
Now compare him to Nani, he came to us and I think he wanted to do too much fancy stuff, the pressure was on him from the start to match Ronaldos flare, but Nani never seems to of perfected the basics, by now he should be solid, he should naturally know his defensive duties, when to run, when to pass, when to shoot etc... But he hasn't, infact, he looks like he's regressed instead of progressed. And I have to be honest, I think it's too late for him to turn it around now, I don't think he will fulfill his potential at all, his attitude and work mentality are just not good enough. I have no doubt we will see glympses of magic from him, but consistently for 90 minutes, 40 games a season? no chance.
My view, I'd sell him on ASAP. Get a good 10 million from him, maybe he'd do well at a smaller club where he plays more often, but he really can't start every week for United with some of the abysmal performances he puts in, you can only give a player so many chances. So, get him out, get Giggs, Obertan, Tosic, Llajic, Welbeck in there more often, or just sign Silva.