Actually he could have. And he has done.
Sunderland at home, for example.
There's a difference between a Premier League side and a League 1 side.
Nani usually excels against defenders from the lower leagues where as he's a bit more rubbish against defenders who know what they are doing.
You can bet your house he'd have given the full backs more of a fight than Obertan and Welbeck did - I can hardly remember them trying to take on a man - something Nani is not afraid of doing.
Oh and he's obviously injuried, I'm not buying all of these conspiracy theories.