Van Gaal's men were far from their best at Old Trafford as Smalling scored either side of Danny Ings' equaliser to give United an undeserved lead at the break.
While Robin van Persie's second-half penalty sealed the points, Smalling explained that Van Gaal left his players in no doubt as to his views on their performance.
"We were lucky to get away with that first-half performance," Smalling told sections of the British press.
"The manager said in the dressing room that the first half was unacceptable and all the players know that.
"I think the manager was shocked at half-time. As players, we knew that it wasn't good enough and we had to stand up and fight in the second half."
United have lost just once since the turn of the year and sit in the Premier League's top three, while also remaining in contention for the FA Cup.
Smalling feels their ability to win when not at their best could stand United in good stead this season.
"It was a strange game in that the first half was probably the worst we've had all season, we were lucky to go into the break with a lead," he added.
"In the second half we fought and played a lot better. If we can win games playing that badly, then it shows that if we do play well and it all clicks, then there will be a lot better performances at Old Trafford, that's for sure."
