Defence seems to be a sore spot for us right now. When you look at teams with extreame pressing systems like us and the masters of it Barcelona the defence is based on aggressive ball winning from the front, every player pushing up behind the next to deny the opposition any time.
This requires players with great tactical discipline, they need to know when to push forward to win the ball back in unison with teammates and when to fall back when the lines are broken, e.g open counter attack.
Sometimes in defensive situations I have noticed certain players trying to press when they should not have and thus leaving space in behind. I don't think our issues generally come from bad individual errors (although we have seen some recently) but mainly from the team shape not always being right and leaving us exposed so buying players won't solve the issue unless we get real quality, I think it's more experience with the system, a few years and we will get it right more often than not.
Pepe Guardiola said of his Barca 'we are a horrible team without the ball' and his Barca were far stronger defensively than us denying the opposition any time on the ball. All pressing teams are vulnerable to the counter and the ball over the top, we don't have Barca's players but we must use their example of how to solve the natural defensive issues that come with pressing and that's far more than get a nice centre back or defensive midfielder.
That's being said I wouldn't refuse Schneiderlin.