We do need to improve our defending, but it's questionable how much of that comes down to whether that involves investing heavily in the CB positions as many people suggest. There's kind of a suggestion that we can either improve our goals haul at the expense of the defence, or we can cover the defence better but offer less going forward. And I think if you're talking about using the same players in a more attacking or defensive way during the season, then there's a certain truth to that. But when it comes to signing players, I think that improving any one area of the pitch actually has a positive effect on all areas of the pitch. If we have three forward players who score over 20 goals a season each, that is absolutely going to affect the way that teams approach games against us and how willing they are to commit players forward, risking leaving their own defence exposed, which means fewer goals against. The rags, for example, arguably have far more talented players going forward than they do defensively, but their attacking players are hindered by their need to cover for what is a fairly average back line (goalkeeper aside).
Obviously this is only part of the story, and great players all over the pitch are what you're aiming for, but I think the "our defensive record isn't great, we need to replace the defenders rather than buying Sanchez" argument is a bit simplistic. Having Sanchez in the team will limit the amount the opposition can or are willing to attack us. Having midfielders that never give the ball away will limit the amount of times the opponents get the ball off us in a dangerous positions with the opportunity to run at defenders. And obviously having better defenders on the pitch will lead to a better defensive record too, but it's not the only consideration. Barcelona under Pep had an incredible defensive record, and I don't think it was because a 21 year old Pique was already world class, it's because no-one else ever got the ball and when they did, they knew that sending too many players forward and losing it would have Messi and co. running at their back line within about 3 seconds and scoring more often than not.
