The Red Devils were beaten 2-1 at the Britannia Stadium as they lost for the eighth time this season in the Premier League.
"I thought we dominated for large parts of the game and on another day the chances we had might have gone in," Smalling told the club's official website. "It was massively frustrating.
"We pressed for the whole second half, we had a lot of the ball but it wasn't going for us. On another day, nine times out of ten, we'd have won the game."
United continue their Premier League campaign next Saturday when they welcome Fulham to Old Trafford.