Arsenal-Chelsea Talking Points  (Sky Sports)

Chelsea dealt a blow to title-chasing Arsenal on Sunday, winning 1-0 at the Emirates after Per Mertesacker was sent off.

Diego Costa proved to be the difference, drawing the red-card foul from the Gunners' defender before netting the match winner in the first-half.

The result leaves Arsenal three points behind Premier League leaders Leicester - and Chelsea's players talking of a seemingly unlikely surge towards the top four.

Here, we round up the key talking points from the game…

Mertesacker sees red

For all of Per Mertesacker's strengths, his lack of pace is a recurring subject for his critics. His top speed on Sunday was bettered by every one of his team-mates - including goalkeeper Petr Cech. With 18 minutes on the clock, his weakness cost Arsenal.

A through ball sent Diego Costa clear and Mertesacker had no chance of making up the ground. After a glance at the linesman - described as "bizarre, stupid" by Jamie Carragher - he launched a lunge at the Chelsea forward. While Costa's theatrics drew criticism from Graeme Souness, the red card produced by Mark Clattenburg was inevitable.

Arsenal, who had been on the back foot since kick-off, suddenly faced an even tougher task - with 72 minutes still to play.

Former Gunners striker Alan Smith suggested on commentary that Mertesacker may have been wiser to have pulled out of the challenge and put his faith in Cech. But, in a split-second, Mertesacker's decision had been made. In five minutes, the red card had been compounded by Costa putting Chelsea 1-0 up.

"This is a major weak point of the Arsenal team," said Sky Sports pundit Carragher. "When Mertesacker gets exposed he hasn't got the pace to deal with it. It's 100 per cent a foul and a sending off."

Wenger's tactical change

Moments before Costa's match-winning strike, Wenger had moved to re-balance his side, following Mertesacker's sending off. But his tactical change drew criticism from Thierry Henry. Wenger opted to replace Olivier Giroud - who showed his dissatisfaction with the switch - with Gabriel Paulista, when Joel Campbell seemed the more likely player to be sacrificed.

This left Theo Walcott as Arsenal's primary striker - before he was substituted for Alex Oxlade-Chamberlain on 75 minutes.

Wenger later told Sky Sports that a desire for pace on the counter-attack was behind his thinking but Henry - who expected Campbell to be withdrawn - said at half-time: "Why is [Giroud] still not on the field? Did you see the reaction of Giroud when he went out? I don't think he was hurt or anything. He could play.

"You could put Mesut Ozil on the left, Theo Walcott on the right, keep Aaron Ramsey and Mathieu Flamini in the centre. Your midfield still looks good and you still have your back four. You're good."

Arsenal only managed one shot on target in the entire game - a deflected Flamini back-heel - and it was the midfielder presented with the best opportunities over the 90 minutes - chances Giroud may well have done better with.

However, Souness sided with Wenger and believes, despite the benefits of hindsight, that the Arsenal boss made the right call at the time.

"[Chelsea centre-backs] Kurt Zouma and John Terry enjoy playing against a big, strong man," said Souness. "They don't want to be taken on by someone who's electric. It's easy to sit here afterwards and say he shouldn't have done that but at the time it was the right thing to do. As it turned out the chances fell to Flamini and that's the way it falls for you sometimes."

Costa gets the better of Arsenal again

"Yet again it's Diego Costa and Chelsea having the last laugh."

Thierry Henry's post-match comment must have captured the feelings of many Arsenal fans leaving the Emirates on Sunday evening. It certainly reflected the view of Wenger, who accused the Chelsea striker of getting one of his players sent off for the second time this season.

There's a sense that, after a sluggish start to the campaign, Costa is returning to his best under Guus Hiddink. His match-winner at Arsenal was his sixth goal in Hiddink's six games in charge, across all competitions. Costa had managed just four in 21 before that.

As Chelsea continue to rebuild, recover and whisper talk of an unlikely tilt at the top four, their top scorer from 2014/15 is emerging from his personal slump.

Fabregas back to his best?

Another Chelsea player returning to form is Cesc Fabregas. The Spaniard bossed proceedings at the Emirates, registering 83 passes in a display far removed from his below-par performances over the past 12 months. Fabregas himself, is feeling good again.

"I'm nearly there, I'm feeling very sharp," he told Sky Sports. "I think my brain is much quicker than in the last few months. I struggled for a little bit. I don't know why. But the sharpness is back. In the last year, I'm in the best form right now, yes."

Fabregas was unfortunate not to be awarded a penalty against his former side, when Koscielny sent him tumbling in the box, but his man-of-the-match winning performance is yet another encouraging sign for Chelsea supporters.

Arsenal's title challenge takes a blow

It was 2011 when Arsenal last beat Chelsea in the league. They've now lost six and drawn three of their last nine meetings. Yet, before this fixture, there was a belief that, with Jose Mourinho out of the way and Chelsea languishing in the bottom half, things may be different.

It wasn't to be and Wenger conceded after the match that both the disappointment of the defeat and the lost points would have an impact on his team's title challenge. "The emotional damage is of course there because of the disappointment," he said. "But what is important is the points, more than the emotional damage. [The title race] will be very tight until the end, with many teams involved."

The result leaves Arsenal three points adrift of shock leaders Leicester, level with Manchester City and just two clear of rivals Tottenham. The Gunners have failed to win any of their past three Premier League games, missing ideal opportunities to make a statement about their title-winning credentials.

After a Valentine's date at the Emirates with Leicester, Arsenal still have to face Manchester United, Tottenham, Everton, and Man City away from home. "We can go back to the top," insisted Cech after the game. Now they have to prove it.