Central defender Smalling was sent off in the first half for two bookable offences as Manchester United went down 1-0 to rivals City at the beginning of November.
Having been booked for a tangle with Joe Hart, Smalling lunged in recklessly on James Milner to pick up a second yellow card, prompting United boss Louis van Gaal to label the dismissal "stupid".
"I really respect Roy. He gave me my chance at Fulham. I was nowhere and he plucked me out and gave me that chance," said Smalling, who started as England beat Scotland 3-1 in Glasgow on Tuesday.
"He didn't need to say much about the sending off because it was very out of character for me and I just have to move on.
"The red card was disappointing but I rarely get yellows so hopefully it will never happen again."
