Arsene Wenger described Santi Cazorla as feeling "half-conscious" in Sunday's 1-1 draw between Arsenal and Tottenham.
Harry Kane put the visitors ahead at the Emirates Stadium in the first half and Petr Cech saved well from Christian Eriksen and Toby Alderweireld to keep the Gunners alive in the match.
Substitute Kieran Gibbs scored his first Premier League goal in over three years to snatch a point for Arsenal, after Olivier Giroud had clipped the bar with a header before flashing another effort wide from point-blank range.
Wenger had limited options available on the bench and his decision to withdraw Cazorla caused some surprise, but he has since explained that the Spain midfielder did not feel well enough to continue.
"It was very intense physically. We were a bit unlucky as well because we played the first half with Santi Cazorla who was half conscious," the Arsenal boss explained after the match.
"I changed him at half-time. I don't know what happened to him. He felt perfectly alright earlier, but when we went to go back out they said he felt dizzy."
While Wenger accepts the result could be considered just, he was left to rue the fact that France striker Giroud was not more clinical in front of goal.
He added: "I have some regrets also, Giroud had great chances in the second half, but overall people will maybe say it's a fair point.
"It was vital not to lose, before the [international] break you can get negative vibes. Manchester City dropped points, we did, but you could have said that it's always likely in a derby, especially after the game on Wednesday night [the 5-1 thrashing by Bayern Munich]."
Wenger was delighted to see Gibbs bag the equaliser just three minutes after replacing Joel Campbell as a second-half substitute.
"It was unexpected coming from the back, but we have very little offensive options at the moment on the bench," he continued.
Gibbs himself acknowledged a strong display from Tottenham which extended their unbeaten league run to 11 games, but credited his own team-mates for refusing to give up.
"They don't come often but when they do it's hopefully enough to help the team," Gibbs said of his goal, a close-range volley following Mesut Ozil's cross.
"Fair play to Tottenham, they showed up and put in a decent performance. We can never give up, we know what it [the derby] means to everyone, to the fans, the players, the club."
