#3 Urgent defensive reinforcements

This seems to be an ongoing trend for years now, and it doesn't feel like it is about to stop anytime soon, as Arsenal have time and again had their defensive instability revealed in moments when it mattered the most.
While Arsenal have had results easier to come by this season than in previous years, mostly due to Unai Emery finally purchasing a midfield destroyer in Lucas Torreira, but luck has also been a major factor, which for a team at this level should not be the case.
Arsenal's starting backline consists of attack-minded Hector Bellerin, the polarizing Shkodran Mustafi, ageing beast Sokratis and soon to be 33-year old Nacho Monreal. Does that look like a title-winning defence? Not in my opinion.
A lot of chopping and changing is required to bring a league title to the Emirates, and reinforcing the defence would be the best way to make a start.