Arsenal had the best defence in the PL last season. Signing defensive players isn’t their problem. They need an elite goalscorer because Havertz isn’t even close to prolific enough. Arsenal need a killer in front of goal because they don’t have one.
They also suffered the least injuries out of the top teams in the league. I don’t think they will be so lucky again. The squad is kinda weak too. The first XI is great but a couple of injuries and all of a sudden Nketiah is starting or Jesus.
While I do agree that they had a great defense, I also think that they lacked depth in defense. I know that may sound weird, but their starters were great, and maybe one more bench is arguably good, but that's it, if anyone gets a red card, and the other player gets injured, suddenly they have literally kids there. This is why getting a bit more depth isn't a bad idea at all, it would benefit them and I would highly suggest something like that.
This is of course not something they should sell the farm for, they should just get someone for 10-20 million range, not some expensive player. And to be fair, aside from getting some good bench players, their starters are all good, these are the players that almost beat City and won the league, saying that you "need" players would be insensitive to the achievement these guys had, I think they did alright. Get like 3-4 players for rotation to make the squad depth bigger, and that's it, I would say try again.
I know that it is going to be a much better season if they do that, because their players would be a lot more accustomed to playing together and that will definitely help with them most of the time, shouldn't be an issue for them at all.