The Citizens had to come back from 1-0 down at half-time, going on to win 3-1 through two Phil Foden and one Erling Haaland goal.
"I didn't look back at it, if I am completely honest, so I don't know," Walker told Sky Sports.
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"I didn't feel that there was much in it. I probably did what I normally do and get my arm across people.
"The referee has seen it, he's checked it on the VAR and they (United) just have to take it as that. We have gone up the other end and it is kind of sod's law, isn't it?"