Chelsea and Arsenal are outperforming Manchester United and Manchester City defensively in the Premier League this season.
There are no concerns from an attacking perspective for United or City, who are both scoring freely, but if their respective managers, David Moyes and Manuel Pellegrini, don't address their defensive shortcomings in January, the title will be won in London.
United went with Rafael, Phil Jones, Nemanja Vidic and Patrice Evra as their back four against Sunderland. Moyes has two attack-minded full-backs there who like to get forward, and that exposes their central defenders on the counterattack.
It's no coincidence that Moyes is still trying to find the right blend across the back. Vidic has had his injury problems, Rio Ferdinand has peaked, and I'm not convinced Chris Smalling or Phil Jones—who has also been used in midfield and at full-back—are the answer.
United need to go out in January and buy a world-class central defender. They need somebody in his mid-20s with international experience who can bring consistency and deliver better coverage in front of David de Gea.
The same is true of City, whose back four consisted of Pablo Zabaleta, Vincent Kompany, Joleon Lescott and Aleksandar Kolarov last time out against Everton. Again, you've got two full-backs there who are very forward-minded and can leave the central defenders vulnerable.
Kompany's injury problems must be of concern, Lescott barely got a look-in under Roberto Mancini, and Matija Nastasic is still two or three years away from being the finished article, in my opinion. City, like United, are crying out for a central defender who can tighten things up for them.
Arsenal, meanwhile, are looking extremely strong at the back. Carl Jenkinson and Kieran Gibbs can both get forward, but they are primarily defensive full-backs. And then you have the hugely impressive central pairing of Per Mertesacker and Laurent Koscielny.
Mertesacker is the real deal. I said as much when Arsenal signed him, and you don't get nearly 100 caps for Germany without being a world-class performer. Arsene Wenger has come upon a fine balance in defence, which could make the difference this season.
Chelsea are also looking very good at the back. Branislav Ivanovic has been impressive, John Terry is back to his best, Ashley Cole's class is undoubted and David Luiz has all the attributes you need to be a top-quality defender.
If I was to rank those four defences right now, I'd put Arsenal's first, followed by Chelsea, City and United. And while it's great to see teams scoring goals and wonderful attacking play lighting up the Premier League, you don't win titles without a strong back four.
When I played for Liverpool, Robbie Fowler and I scored 55 goals between us and combined for countless assists. But we didn't win the title because we lost games against lower clubs and conceded too many goals when it mattered.
If United and City don't act in January, they may be telling the same story come May.