If you're down to 10 men and playing predominantly on the counter, is it not better to have the pace of Theo Walcott up front? It seems most of you think not but I can see the logic.

Chelsea look for a second as Cesec Fabregas weaves past a challenge on the edge of the box but his low shot is well blocked. 

neil leverett: Gotta leave the target man on Wenger. Giroud may be injured tho

Imy Hussain: Now know why Mertesacker gave Theo Walcott captain's armband..knew he wouldn't be there the full 90...concert perhaps? 

Scott Thomas: It's situations like this you need a cool headed manager. Unfortunately Arsenal have Wenger who takes off Giroud. 

Jay: Leicester might not have to worry about the goal difference after all! 

Here's a look at the goal which separates the sides. Mark him up!

Image source, Reuters

Image source, Opta

John Hartson
Former Wales striker on Radio 5 live

"I could see Arsenal still being in the game with Olivier Giroud, he is the focal point, he can get hold of the ball, if Arsenal put crosses in he is more likely to win them. I can not understand the decision to take him off.

"Arsenal now have a mountain to climb, Chelsea have an extra man and a one goal advantage."

Phil McNulty
BBC Sport chief football writer at The Emirates

Chelsea striker Diego Costa is already a villain around these parts after he provoked Arsenal defender Gabriel into a red card at Stamford Bridge earlier this season.

Costa also received a retrospective three-match ban after a clash with Laurent Koscielny in that game - and he is at the centre of the action again.

Arsenal can have no serious complaints about Per Mertesacker's red card for hauling down Costa but now he has added insult to injury with that goal.

He is getting right under the skin of Arsenal's fans - who were furious when manager Arsene Wenger sacrificed striker Olivier Giroud to reintroduce Gabriel to Costa in his reshuffle after the red card.

That's a chance for a second for Diego Costa as the Arsenal defence again lose track of him, this time the Spaniard/Brazilian can't control a volley at the far post and it slices off his ankle and away!

Alexis Sanchez is out warming up, which has got the crowd onside if nothing else. Chelsea doing their best to keep hold of the ball at the moment, make that extra man tell.

Matt Graham: Arsenal against Chelsea is like Groundhog Day. Arsenal always self destruct.

Adesola Ibikunle: Wait...what's Arsene doing? Giroud out? That's the end...

Charlene Beedie: How on earth can Arsenal be title contenders? 

David Thackeray: Bad move by Arsene. Diego Costa will provoke Gabriel.

Hector Bellerin wins a corner as a shell-shocked Emirates crowd tries to compute another Diego Costa show. Did he dive in the red card incident? I think the fault lies squarely on the large, Germanic shoulders of Per Mertesacker but the striker did make a meal of it. He took his goal superbly though.

This is all potentially good news for Leicester City of course...

Image source, Reuters

Michael Burrell: Is there a safer bet than Arsenal getting a man sent off against Chelsea? 

Chiemezie Onwubalili: Not a red card. there was no contact, and that's coming from a chelsea fan

Theo Bowyer: Mertesacker's lack of pace exposed there... Definite red card

It took Gabriel a matter of seconds to lose track of Diego Costa as he meets a cracking cross from Branislav Ivanovic to sweep in at the near post. He looks a different man under Guus Hiddink!

John Hartson
Former Wales striker on Radio 5 live

"No complaints from me, Per Mertesacker brings down Diego Costa when the striker is clean through on goal. His inability to mark Costa properly allows the Chelsea player to run in and has got him a red card."

Well this hasn't gone down well! Olivier Giroud is the man replaced as Arsenal slot centre-half Gabriel into the action. Giroud does not like it, the crowd do not like it...

All three of Arsenal's league red cards this season have come against Chelsea! It's the Diego Costa effect...

If Per Mertesacker is the last man, you always sense trouble! The big German is last man as Chelsea counter, Diego Costa toes the ball past him and goes down. Referee Mark Clattenburg has a long, hard think and then goes for the top shelf.

RED.

Did Mertesacker actually make contact? Costa was away if so. 

Chance! Chelsea's winner, and runner-up, of all of their player of the season awards, Willian, has a volley at the back post blocked and then he toe-pokes the rebound on goal. Petr Cech makes the save.

Where's the Flamini masks?

Chelsea having a spell of keep ball now to try and take the sting out of this one. 

Incredible scenes...