The 26-year-old was given his marching orders following a second yellow card with 15 minutes left in extra-time of the final against Crystal Palace, but eventually, United claimed the trophy thanks to a superb strike from Jesse Lingard.
Smalling said: "I think the over-riding emotion was one of relief.
"You just have to learn that you can't always go and win that ball and there are certain moments to do it.
"But luckily I wasn't punished and Jesse popped up with a great goal. But I thought it was a well-deserved win because we created a lot of chances."