Arsenal 0 Chelsea 1: Five things we learnt, including Per Mertesacker is the weak link in Arsenal's def

A Diego Costa goal secured the points for Chelsea after Mertesacker's red card

Per Mertesacker is dismissed by Mark Clattenburg (Getty Images)

Mertesacker is the weak link in Arsenal's defence

Having a strong back four is vital for any side that wants to win the title, which is why Arsenal fans should be concerned about Per Mertesacker.

The German international certainly has a lot of experience of lifting major trophies - after all he won the World Cup with Germany in 2010.

But his lack of pace always makes him vulnerable and so it proved this afternoon against Chelsea.

Granted Willian put Diego Costa through with a fine pass, but a faster centre half would not have had to resort to such a desperate lunge.

Costa is not the quickest, yet because of Mertesacker's subsequent red card, it put Arsenal at a distinct disadvantage.

Forget Cesc Fabregas, Diego Costa is the Chelsea player Arsenal fans hate the sight of most

In the popularity stakes at Emirates Stadium, it would be fair to say Fabregas and Costa can never expect to be given a warm welcome.

Given how Fabregas is perceived to have betrayed his former club by joining Chelsea from Barcelona in 2014, one would think he would receive the worst reception of the two.

Yes there was some booing at times, but it was Costa that the home fans clearly loathe the most and they would argue with good reason.

It was Costa who wound the Arsenal players up to good effect, in Chelsea's 2-0 victory in September, which led to Gabriel being sent off. He was later handed a retrospective ban by the FA for slapping Laurent Koscielny.

The 27-year-old came back to haunt Arsenal again but for all the right reasons on this occasion.

Firstly it was his run that led to Meretsacker's red card and then he scored a fine goal at the near post to secure the points.

Walcott won't remember his 10th anniversary fondly

This was supposed to be a happy occasion for Walcott as he celebrated 10 years in an Arsenal shirt.

Such was Arsene Wenger's desire to mark the moment, he handed Walcott the armband rather than normal choice Mertesacker.

Given Chelsea's slovenly backline, the game should have been a perfect opportunity for him to use his pace to good effect.

But the England international was a huge disappointment, first out wide and then in the middle after being moved there following Mertesacker's red card.

Cech still looks a better keeper than Courtois

Chelsea had a very tough decision to make in 2014 over who was to be their first choice keeper for the future.

In one corner was Cech, the veteran who eventually won 13 major trophies in 11 years and Courtois, who'd spent three seasons on loan at Atletico.

It was inevitable one would leave if they were made the back-up and as Courtois was 10 years Cech's junior, he made way and last summer Arsenal took advantage.

Whereas Cech has shone this term and did so again this afternoon, keeping Chelsea at bay on several occasions, his opposite number was suspect.

His uncertainty almost led to an equaliser on a couple of occasions when he flapped at a cross, which led to an ugly goal-mouth scramble.

Courtois is clearly one of the best keepers in world, but's he's not as good as the man he replaced just yet.

Are the two clubs heading in different directions?

Sounds ridiculous given their respective positions in the table, but Chelsea's continued dominance over Arsenal could have a huge bearing on what happens for the rest of the season.

Chelsea are now eight games unbeaten under Guus Hiddink, but this is clearly the most significant result of the lot and will give the players belief they can still rescue something from the campaign in the FA Cup and Champions League.

Conversely, Arsenal's failure to end their five-year drought with a win against Chelsea in the League will raise questions again over their ability to lift the Premier League trophy.

Two points out of the last nine available is not what champions are made of.

In 2014, Chelsea won at Liverpool to put a major dent in their title hopes. This setback has come a lot earlier in the campaign, so at least Arsenal still have time on their side to put it right.