Of course he is a big issue, he isolates himself to a greater degree via poor movement and positioning. Added to that when he does get the ball, his touch is poor and he does nothing with it. You put a top class player up top and they would get many more touches and be much more involved. The best strikers often find goals even in poor teams, hence why they cost so much. Too many people speak as if a CF is just there to eat what is served on a plate for them, as opposed to actually having any sort of clever movement or link up play. CBs love it when a CF gets tight to them, it means they know exactly where the player is and can feel where he wants to go and what he wants to do and have a big say in that outcome - the best forwards mix it up and move arouund away from CBs dragging them around into uncomfortable positions, make them question whether they follow or stay where they are while they also have to keep an eye on where the ball is and any other potential runners. Instead, Hojlund cuddles them so they know that the ball may not come, as he is tightly marked and if the ball does come his way it is straightforward to deal with and any other runners can be dealt with by teammates as they are not going to run through the zone that Hojlund is hugging them in and make them decide whether to stick with Hojlund or leave him for the runner.
It was clear from early on that this was an issue, there was a time when Hojlund had touched the ball less than Martial in twice the amount of minutes - that highlighted clearly that the player himself is a huge reason for that lack of involvement. Whenever people talk about Hojlund positively, it is clutching at straws - 'he had one good run', 'he had one good shot', 'he was in a great position one time and didn't get picked out'. We used to find a league top scoring Berbatov, a disappointing striker!
Even if we had signed a young Wayne Rooney, it would be ridiculous to expect him to be the lone CF with no other quality around him let alone a player who had less than 10 league goals in a top league - baffling decision making. Players with more hype like Vinicius were brought in for big money to learn behind actual quality like Ronaldo and Bale, not immediately plopped in. Ultimately, Hojlund cost wonderkid money and is our starting 9, he is nowhere close to good enough for that role or status. Maybe some day he will be good enough, but he is far from it currently and that needs to be addressed as soon as possible.