Mikel Arteta has revealed midfielder Declan Rice was brought off at half-time of the 2-2 draw with Tottenham due to a back problem.
England international Rice played the first 45 minutes as Cristian Romero's own goal and Son Heung-min's first effort sent the two sides into the break level.
However, Rice did not reemerge for the second period of the north London derby, with Fabio Vieira also making way as Jorginho and Kai Havertz were introduced from the bench.
Explaining the substitutions in his post-match press conference, Arteta revealed: "Declan is not feeling good, he felt something in his back and could not carry on, and Fabio was a tactical decision.
"We had to adapt to it and try in the second half. We scored a goal and it was a huge blow when we conceded straight after."
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The Gunners had retaken the lead early on in the second when Romero was punished for handball, allowing Bukayo Saka to cooly clip in from the penalty spot, but Jorginho was caught in possession by James Maddison before Son slotted in his second.
One Gabriel Jesus chance could have put Arsenal 2-0 up in the first half and Arteta was rueful of some of the missed opportunities that cost his side the chance of scooping all three points.
He added: "Well especially when you go up twice, and the moments and the chances that we didn't make, especially in the first half.
"We wanted more but you have to make the most out of it and we didn't. The moments where we had dominance of the Game and control, we conceded the goals and that was a big, big blow.
“Playing in front of this crowd in a derby doesn't get much better than that, and I think the players were so excited but sometimes you can be too hyped because then we lacked some composure on the ball to get more control, but we have to take the draw, there is nothing else we can do."
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