Two goals up at halftime, City played almost the entire second-half with ten men after Rodri's red card.
"It's not ideal going down to ten men," declared Walker. "The manager made the changes and we have to adapt to that.
"It's something we have to do. It's about getting the three points and moving onto the next game.
"There's emotions running high in the game. He [Rodrigo] isn't one to start raising his hands so I was surprised to see the red card.
"I see it in a quick flash with no replays, so the referee has made his decision and we have to stand by it. As a person and a human, I don't think he has that in him."