The Manchester United hitman has already notched 22 goals this term - with 10 in his last 10 games.
But the £22million summer signing from Arsenal declared: "It should have been 10 games, 14 or 15 goals. At least four or five I shouldn't have missed.
"It's not going really well. It is going well but if you start to believe it's going well, you're going down.
"If I look at all those games, I should have done better in most of them because I had more chances than just one chance and one goal."