Arsenal had to react and they did. It was Merino who scored the equaliser, a tribute to his own tenacity as much as anything else.
And it was Merino towards the end who did almost nick the win, his low shot well saved by Robert Sánchez. The Chelsea goalkeeper jumped up to block bravely on the rebound from the Arsenal substitute, Viktor Gyökeres.
And so Arsenal did depart with a point. It might have been more had Jurriën Timber not taken an aerial ball away from Gyökeres at the very last but nobody could say that it was not a fair result. And a pulsating contest.
Hands up who had it marked as a top-of-the-table showdown at the end of September? That was when Chelsea had just lost at home to Brighton to sit eighth and the rumblings of discontent were audible. The upturn in their fortunes was highlighted by the Champions League dismissal of Barcelona here last Tuesday and the Saliba news had to be a boost for them.
The word from Arsenal was that the injury is not too serious but it was serious enough. With Gabriel Magalhães already missing, it meant an acid test for Cristhian Mosquera and Piero Hincapié in the centre of the Arsenal defence.
This is a derby with plenty of snarl and it was only eight minutes old when Bukayo Saka could be seen telling Timber to keep his focus; Saka pointed at his own temples to make the point. Timber had been bundled over by Caicedo and by then we had seen Mikel Arteta’s first eruption after a Marc Cucurella foul on Saka and the first yellow card – Martín Zubimendi on James. There would be more. Three of the Arsenal back four were in the book by half-time, Timber the exception.
It was always going to be a difficult occasion for the referee, Anthony Taylor, albeit he was a unifying figure on one level. Both sets of supporters see him as a villain. Or words to that effect. There were other descriptions aired about him at various times.
Taylor had no option over the big call of the game, particularly when it was put to him by the video assistant referee, John Brooks, that he ought to take a closer look at Caicedo’s challenge on Merino. It was high and dangerous, a potential ankle-breaker, the definition of excessive force. Caicedo did slip slightly. It was, nevertheless, a shocker.
Up until that point, Chelsea had shaded it. They bristled with intensity and they created a few chances, the biggest one for Estêvão Willian on 18 minutes. It was a poor clearance by Mosquera after a Pedro Neto cross. Estêvão blazed high. Moments earlier, Declan Rice had been required to stretch into a saving tackle on Neto. A little later, João Pedro hesitated after a slip by Hincapié. He should have shot first time.
It said everything that the first half ended with Enzo Fernández seeming to swing an arm at Mosquera, leaving his opponent prone as the players headed for the tunnel. Arsenal wanted to regroup, to work out how to make the extra man count. Saka had gone close in the 12th minute after a velvety touch and pass from Eberechi Eze while Gabriel Martinelli extended Sánchez just before the break.
It was Chelsea who bolted out of the blocks upon the restart. From a James free-kick, João Pedro forced Raya to tip behind. From the corner, Chalobah rose above Declan Rice to flick on for the far corner. To unbridled home delight, the ball looped home.
Maresca did not make negative changes after the red card. He stuck with two wingers and a striker. That said, it helped to have James playing like two men in midfield. The captain was immense; a remorselessly driving presence and creative, too. Some of his passing was of the highest order.
The Arsenal equaliser had not been advertised. Saka, though, is always capable of making something happen out of nothing. He ran at Cucurella, who had been booked for going through him in the 11th minute and, when the winger checked inside, his whipped delivery was on the money. Arsenal had two against one at the far post. Merino rose imperiously.