I agree but it is one of the less under-invested positions.Even then we "invested" in Trippier (sold as per Poch) & Aurier (Picked by Poch)....
That's not a spending issue.
The only other position I think you could argue is up top but how do you get in a decent striker to sit on the bench for 85 mins per match? Plus we've seen Son play as a false 9 before Llorente was used so I'm not even sure we are in need.
This whole under-investment thing is a myth in my opinion. We've seen big money moves made for numerous players in our weakest positions. If Poch doesn't want to play Foyth or Sanchez ahead of an ageing and non-committing Toby/Jan then that's not Levy's fault.