The right-back was given his marching orders just before the half-time whistle after receiving two yellow cards from referee Bobby Madley inside three minutes.
After the match, when asked by Sky Sports News for his reaction to Walker's sending-off, Guardiola said: "Next question please."
On the game itself, the Spaniard said: "The second half we made a good performance, similar like last season when we played them here. We created the chances in the first half but we did not finish them. We have to try to create the chances.
"They made a good action. We have done everything, I don't know how many chances we created or how many they created. In the second half we made some tactical changes and created chances.
"I am pleased with all of them. We lost two points. We are here for the results, at the end we have to try to analyse the performance. Ten against 11, a Europa League team, we tried. We missed some chances at the end. Even though we scored we could not win."
City have collected four points from their first two games of the season.