The England full-back was shown a straight red card for a needless hack at the legs of Leipzig goalscorer Andre Silva after 82 minutes of City’s 2-1 loss behind closed doors at the RB Arena.
With top spot in Group A already assured, City had little to play for in their final match of the first phase but the suspension that will now follow for Walker could be costly in the next round.
Guardiola gave short shrift to questions about the incident in his post-match press conference.
“It’s a red card, definitely,” the Spaniard said bluntly. “I’m not coming here to say what happened in the dressing room.”
Guardiola had no complaints about the result. Leipzig, who had gone into the game in crisis after the sacking of manager Jesse Marsch last weekend, won with goals in each half from Dominik Szoboszlai and Silva.
Riyad Mahrez pulled one back for City 14 minutes from time but, after Walker’s dismissal, a comeback proved beyond them.
Guardiola said: “In the first half we lacked rhythm. We had problems in contact and they have a really good team – we knew that from before.
“The second half was much, much better. We were more aggressive, we played to win the game but, unfortunately, we made one or two mistakes and they punished us. We lost. Congratulations to Leipzig.”
City, who reached the final last year, have not kept a clean sheet in their last five matches in all competitions but, having travelled to Germany on the back of seven successive wins, Guardiola was not overly concerned.
He said: “Of course it is better not to concede goals but in all the games we played much better than the opponents, except today where we struggled a little bit in the first half.
“But in general we made an incredibly good group stage. We qualified with one game left from a tough group, so I am more than satisfied with the performance in every single game.”
Of more concern to Guardiola might be the fitness of Phil Foden and Nathan Ake, who were both substituted with injury issues.
Guardiola said: “Nathan has a problem with his back. We will see tomorrow. Phil had problems in the beginning of the game with his ankle. He struggled a little bit so he was taken off as a precaution.”
The night belonged to Leipzig, who shrugged off their recent problems to secure third place in the group and a Europa League spot.
Interim manager Achim Beierlorzer said: “We had to show a reaction. This game was perfect for us because we proved something.
“We took chances and could have had more. We beat one of the best sides in the world.
“It is a pity there were no fans here but we did a great job. It was a highly-deserved win.”
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