lol, sure, Gomez and llorrente are relatively tall, when Spain and Bayern play all they do is play the long ball, they certainly aren't passing teams, sure.....

Rooney scored headers that one season for one very simple reason, he played as the furthest forward player ready to get on the end of crosses. Hernandez(when onside) is that player at the moment but, is rather meh, rarely onside and too small/weak to win headers.
You don't have to be tall to win headers, the big tall idiot players clubs get in, Crouch, Zigic, other numpties, I wouldn't put a single one of them in the top 50 best header scorers in the world. Messi scored a header against Utd in a crucial game. Positioning and the cross mean far more than height.
Hernandez is a one trick pony and offers nothing else when that trick doesn't work. You want to buy a player to fill a one trick role, and won't be good enough when asked to do other things either?
You simply need a quality player, the reality is you should be slapping Rooney around the head in training everytime he drops towards midfield till he stops doing it then playing an actually quality playmaker/withdrawn striker like Berbatov behind him and do this consistently.
But, honestly Utd would be mental to sign a striker before a couple central midfielders.