From ETH point of view, of course he needs to defend Weghorst. However, ETH is creating a problem for himself by starting Weghorst.
Assessing the third goal first to get it out of the way. Weghorst made a run that professional no 9 should be expected to do. It's tactically well done. However, not irreplaceable in whatever sense. I doubt ETH mean "a target for link up" as focal point of attack because our players don't see him as reliable passing options with how often they ignore a pass to him as we saw in this case too. David Luiz regen made a serious case of him coming close to the real one in headless chicken department. His partner tried to hold his position to play an offside, while he freaked out and followed WW, left a hole middle. Next this guy was drawn out by both Sancho and Bruno. When the ball came back to Sancho to score, he was lured way out of position that embarrassing is understatement.
In another word, IMO he made unforced mistake that help us more than Weghorst basic movement, position did. Weghorst movement at the end of this 3rd move is why he's not scoring more for us: The space was opened up for Sancho. Weghorst had better chance to be receive a pass if he stayed for a ball at far post, behind the CB instead of moving in front of the remaining Leicester CB. It doesn't not count as occupy the CB, because this CB would unlikely move from his point in that situation. So nobody likes to pass to someone who is marked, while they have another free option like Sancho. No amount of pointing to his feet would change that.
While Rashford is important for his victory, I would say this Faes guy is equally most influential in all the sequence leading to our 3 goals.
1. First goal: Unforced mistake, giving possession away needlessly from Weghorst's header. Then positioning weirdly, leaving Rashford freedom to run and take his shot.
2. Second goal: Positioning himself for the offside trap poorly.
3. Third goal: as explained, he was sold by everything, while leaving his defensive zone wide open.
Bonus: Also lost Dalot for his missed sitter (only has one player to mark. Barged into his teammate which slowed himself down in that sequence of play too).
Back to Weghorst, he had a decent game here where his positive edged out his shortcomings. Getting better as Leicester lost the plot. However, when the score is closer, or in the first 20 minutes where Leicester is up and running, Weghorst is almost tactical nobody on the pitch. The pace was too fast. His so called pressing was ineffective, our midfield was outnumbered. He couldn't help tracking runner. He couldn't hold up, or offering an outlet as a runner. If DDG failed to save all those shots until we killed the game, it would be a much more difficult game tactically.
If anything, Weghorst's luck is at all time high. He's benefited from just being there where Rashford hit peak form, Xavi decided to mess with his defensively line up, Faes having a mare, our players injured suspended...