The Red Devils cruised into the quarter-finals with a 4-0 success at Old Trafford, but had a helping hand from referee Kevin Friend after he pointed to the spot in the 20th minute following Leroy Fer's innocuous clash with Adnan Januzaj.
Javier Hernandez stepped up to convert from the penalty spot and then doubled United's advantage with a header early in the second half.
Phil Jones volleyed the third and substitute Fabio completed the scoring as the game went into 11 minutes of stoppage time following a worrying head injury for Robert Snodgrass.
Round told Sky Sports: "I didn't look like a penalty to me, it looked like he slipped over, and it was very harsh on Norwich.
"In the end I suppose it didn't really matter because the other goals made it comprehensive.
"He (Hernandez) is an infectious player. He loves to score, we see it every day in training when he buzzes around the place.
"He had a good chance at the death with a header to make it three and get his hat-trick, but I'm sure he'll take two and move onto the next game."
United have now gone six games unbeaten after a difficult start to the season and Round feels momentum is building for David Moyes' men.
He said: "This club is used to winning, it has a winning mentality. There were no great celebrations after the win on Saturday and none tonight.
"We just move on to Fulham, that is how it is. You need to go on runs in this league and this club is the best in the business at building that."
Hernandez added: "I don't want to speak about the referee. They are human, they are like us, we can miss a pass, they can miss a decision.
"I don't know if it was correct or not because I didn't see it. The referee gave it and I took the penalty and thankfully the ball went in."