Defence was a bit of an issue in both games against Spurs, Fulham, Villa, two of the three games with West Ham, against Chelsea, against Liverpool, against Lens, against Bayern... yeah. I don't really get people complaining about us signing Calafiori.
Where you spend money on the team is not a simple calculus of where the squad is weakest (though I'd argue LB was a pretty significant weakness last season). Who is available, how they compare to the existing players etc should all factor in.