Defending isn't just about clearing the ball and winning headers, there are so many physical aspects as well as psychological. His positioning is not great, he always seems to be stressed and on the back foot. A lot of clearances for throw-ins because he doesn't know where the striker is or the position of our nearest player. When we let in goals with him on the pitch, he is often "just" behind the one that scores. Rio Ferdinand was a cool and calm head, he seemed a bit nonchalant - but with good reason. He was always ahead of the striker, excellent positioning skills, seldom had to tackle 50/50 and pick up yellow cards as a consequence. You also have someone like van Dijk, who thinks he's as good, but often find himself in problems because he thinks he has better control than he really has - the situation against Sterling being the best example right now. If he is less nonchalant, that's never a goal or a chance to begin with. Lindelof is on the opposite side, he seems stressed and erratic, like his life is in danger for 90 minutes. It doesn't bode well with the others, they don't trust him and his body language - it's basic trust 101 for anyone that has played as a defender at any level.
So far I haven't seen a player in Lindelof. Sure he is playing and doing reasonably OK, but this is Manchester United, not Newcastle or Stoke. We need defenders that scare the other team's striker. Kane and Aguero aren't going to feel sick when they see him in our line-up, quite the opposite. We shouldn't judge a player based 6 months, but we also shouldn't have blind faith in the notion that every single player on the planet might become a superstar. That's just naive; we are Man United and the required level is extremely high. Right now, Lindelof is not even close.