#3 Per Mertesacker
Per Mertesacker might be slow and he might be mocked a lot for that, but one can't really deny that he does not possess the other qualities one looks for in a defender. The German is set to retire at the end of this season and begin a career in coaching at the Arsenal academy.
The centre-back might not have played a lot in the last 2 seasons, but his replacement just cannot be Callum Chambers and/or Rob Holding. The two are just not good enough and Arsenal need to spend some money and get someone – one with the personality of a defender – not someone who's just there because he's been told to be there.
Stefan de Vrij is available for a free transfer, but in my opinion, it could either be Manchester United's Chris Smalling joining them for £25 million this summer, or maybe even Chelsea's Gary Cahill.