Is that a big problem?
Trying to remember if Aguero as an example has headed that many?
I don't see it as a problem at all, it depends how we train our attackers and what we do with the ball in the final 3rd. If we're telling them to whip it in for Martial, Rashford and Greenwood to attack at head height, we're not going to get much luck (unless it's through clever movement like Rashford vs Villa) as most PL CBs are over 6ft and are strong in the air, they're well drilled from an early age about defending in the air.
However, if we're intricate with our build up and have the ball whipped between defender and 'keeper or pulled back both along the floor, then we'd have more luck as these CBs aren't as agile as most forwards, hence why Liverpool, City, Chelsea and Leicester and ourselves are scoring so freely, as teams are playing to their forwards strengths in and around the box and in the build up (like our fast counter attacking approach).
This is why Martial was a better alternative to our gameplan than Lukaku, feed him the ball (as he's got a great first touch) and he can either feed it to another player on the counter, thus committing a defender, or he can turn his man and run away with the ball.