Yes, recruitment, intelligent recruitment will be a big determining factor here. Often it can take a few windows to cycle through some players to get the right fit. Look at Pep with City, that even took time and he had an endless sum of money to spend, our very own Klopp took a few seasons and some players that maybe weren't at the required level. Are you at all concerned about the decision in the summer to prioritise spending all that money on Pepe over some quality defender(s). To me that seemed a really bad decision and I'd be concerned if that person is still at the club. Is that Edu? Obviously how well he works with Arteta and Arteta's ideas will drive a lot of your success on the field.
With the Copa America being on, i don't think Edu was 100% involved that last window. I would imagine now him & Sanllehi will be the main driving forces with Arteta's input of course. As for the defensive situation, i don't think it was totally planned out that way. I think firstly they wanted Saliba to come in straight away, but eventually agreed to St Etienne's request to let the player go back on loan for the year.
Secondly, i think they underestimated Koscielny's desire to leave. I think they thought they could get 1 more year from him then allow him to leave next season. Hence, Luiz being signed late in the window after funds were spent elsewhere.
We've been unfortunate with Tierney getting injured again, & obviously Bellerin is taking a while to get to full fitness.
Apparently the thinking behind Pepe was to try & make us more dangerous in the away games. Our home form was fine generally, but we struggled away from home, as apart from Aubameyang we had no pace in the team. I think the idea was to be more compact, & to be able to hit on the counter more. However, the players struggled with Emery's tactics, having Torriera playing so high up the park sometimes & having Xhaka as sole defensive midfielder was madness, he chopped & changed personnel & formation constantly, & the players looked lost & confused.
Under Emery, i'm not sure another good defender would have made that much difference to be honest, as we left the defence exposed time & time again. Under Arteta though, we look tactically better. We now need to improve some personnel within it.