I'm not quite as down on Lindelof as some on here are. I actually think he's a really good reactive defender. He times tackles and blocks well, he's good at managing opposition runs in behind, and he's adept at carrying the ball under pressure from opponents in addition to be a pretty fantastic long-range passer.
The issue, which many have already picked up on, is that he is absolutely hopeless when he needs to defend proactively. He has an awful tendency to step in front of the defensive line but not actually do anything to make it difficult for the man on the ball to complete a pass or intercept the pass when it has been played, which then leaves gaps in behind. Furthermore, he doesn't assert himself nearly enough in duels. It's not inherently bad for a centre half to jockey the attacker rather than immediately lunging for a tackle, but Lindelof tends to fail to close off the attackers passing/shooting option even when the defence has regrouped.
A lot of people last season thought it was Maguire who would struggle to adapt to Erik ten Hag's defensive system but I actually think that Lindelof's weakness as a proactive defender will probably hamper him even more, since it is the quick restriction of space which comes with proactive defending that forms a key facet of Ten Hag's teams in the transition and defensive phases.