I think I know what @Claudius is getting at when he says we don't just need better players, we need more bodies, since I am of a similar mindset. I obviously don't want us going out to get squad players but I think we get too caught up in if player X is better than our current option. We spent the latter half of this season looking at a bench that often consisted of numerous left backs, Nwaneri and then filler youth players. And that was with a CM playing at 9 already. If we can end this summer having brought in a handful of players that are going to, at the very least, compete for starting spots in our squad and thus have very little difference in the quality of our bench vs the quality of our starting XI then I think that is a successful summer.
For example, if we bring in Sesko and Havertz is still our starting 9 at the beginning of the season, I don't think that's a bad thing. We now will have a very talented, young striker to bring off the bench when needed and he won't have the weight of the world on him to produce from day 1. The same applies to Martinelli. Personally, I still feel as though our first choice XI is title winning quality (making it hard to find players who are definitely better), but we don't have the depth to be able to absorb any injuries, loss in form or rotation when needed in a congested schedule. Give Arteta a group of 18-20 quality players that he feels confident in relying on and we will be able to adjust all season to whatever injuries, form or fatigue throws at us.
