The 21-year-old scored 23 goals for United last season.
He said: “I’ve been working on my heading a lot in training, just taking crosses and working on my timing.
“I’m not bad in training but then the games come and . . . I dunno.
“I’ve only scored one header for United and I haven’t a clue why. But, hopefully, I’ll get more this year.”
He added: “There’s more to my game than just scoring. I went 12 games last season without a goal but I set them up for others and we were still winning, so it didn’t bother me.”