Arsenal will play Premier League leaders Manchester City in the final of the Carabao Cup thanks to Antonio Rudiger’s own goal and a deflected assist.
Chelsea took the lead in the 4th minute when Eden Hazard finished with his left foot past Ospina who was in goal.
But the Gunners needed only four minutes to get level with their London rivals.
Monreal’s header from a corner kick was first deflected by Marcos Alonso then Rudiger, who beat Willy Caballero in goal.
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Chelsea looked the better side in first half but a shadow of themselves in the second, as Arsenal stepped up their game.
Arsenal boss Arsene Wenger said his side were playing with “more clarity” following the transfer of Alexis Sanchez as the Gunners came from behind to beat Chelsea and set up a Carabao Cup final with Manchester City.
After a goalless first leg, Eden Hazard put Chelsea ahead before an own goal from Antonio Rudiger levelled it, and Granit Xhaka’s winner took Arsenal to Wembley.
It came only two days after Sanchez’s drawn-out transfer to Manchester United was completed, with the Gunners getting Henrikh Mkhitaryan in return.
A miserable couple of months for Arsenal has seen them drop to sixth in the Premier League, 23 points behind leaders Manchester City, and lose to Championship Nottingham Forest in the FA Cup.
But now this vital victory keeps them on course to win their fourth trophy in five years – and with it claim the only domestic competition that Arsene Wenger has not won in his 21 years at the club.
“I felt that in January we paid a heavy price for uncertainty in our dressing-room,” said the 68-year-old. “Now we are back with more clarity. We know what we have to do. We can focus on our game.
“We lost a great player in Alexis Sanchez but when a team doesn’t know what’s going on in the dressing room there is less clarity and focus on performances. We know we have to deal with it now.”
The Gunners, bidding to win this competition for the first time since 1993, meet Manchester City in the final at Wembley on 25 February after Pep Guardiola’s side saw off Bristol City in the other semi-final.
With supporter unrest at Emirates Stadium growing in recent weeks, the home fans were left fearing the worst after only seven minutes when Tiemoue Bakayoko played the ball into Pedro and he released Hazard, who slotted past David Ospina from 12 yards out.
But rather than collapsing, the Gunners responded with one of their best defensive performances of the season – and gained two slices of luck with their goals – to move into their first League Cup final since 2011.
Chelsea, third in the Premier League and undefeated in 12 matches in all competitions, only managed one more shot on target and Arsenal equalised within five minutes as Nacho Monreal’s header bounced off Marcos Alonso, then Rudiger and into the net.
There was also a touch of fortune about the Gunners’ second as Alexandre Lacazette’s pull-back rebounded off Rudiger and Xhaka pounced from close range.
Arsenal might have added a third but Alex Iwobi’s shot was blocked by the legs of Willy Caballero – however, the miss did not prove costly.
The Gunners, who saw a £43.6m bid for Borussia Dortmund striker Pierre-Emerick Aubameyang rejected earlier on Wednesday, held out to claim the win as they look for their first League Cup success in 25 years.
Chelsea’s ongoing pursuit of a target man has seen West Ham’s Andy Carroll, Stoke’s Peter Crouch and ex-Manchester City striker Edin Dzeko all linked with a move to Stamford Bridge in this transfer window.
And with Alvaro Morata out with a back injury, Antonio Conte’s lack of imposing options in attack saw the Blues boss opt for a pacy and diminutive front three of Pedro, Hazard and Willian.
The selection paid off inside seven minutes when Pedro set up Hazard for the game’s first goal, but a hamstring injury suffered by Willian later in the half forced an alteration, with England midfielder Ross Barkley introduced off the bench for his Chelsea debut.
The £15m arrival from Everton was making his first appearance of the season after hamstring surgery but was immediately in the action, first being heavily challenged by Jack Wilshere before escaping a booking after a clumsy foul on Laurent Koscielny.
Barkley started on the left, moved over to the right and later appeared in the middle but could not make an impact and, unsurprisingly after such a lengthy absence, struggled for fitness. However, his late corner did give Rudiger a chance but the German defender headed over.
Without an England appearance since May 2016, Barkley will have a lot of work to do in the second half of the season if he is to force his way into Gareth Southgate’s World Cup plans.
Meanwhile, it was another impressive night for one of Barkley’s rivals for a berth in the England set-up, with Wilshere – no stranger to injury setbacks himself – continuing his fine form in the Arsenal midfield.
