Smalling scored a brace and Robin van Persie converted an 82nd minute penalty on Wednesday as United beat Burnley to move up to third in the Premier League.
Van Gaal’s team have lost just once in 18 matches but a section of United’s fan base is becoming dissatisfied with the team’s style of play, which is often laboured and unproductive.
That was certainly the case against the Clarets, who passed the ball better and looked more incisive at Old Trafford in the first half even though their team cost a fraction of Van Gaal’s expensively-assembled starting XI.
Alex Ferguson was renowned for his angry half-time talks when not satisfied with the Red Devils’ performances and on Wednesday Van Gaal was unhappy with his team at the break, and the Dutchman made his players acutely aware of his feelings in the dressing room.
“I think the manager was shocked at half-time,” Smalling said.
“The manager said in the dressing room that the first half was unacceptable and all the players know that.
“It wasn’t a 3-1 scoreline game. Especially in that first half, we all let ourselves down. We could have played much better.
“That the first half was probably the worst we’ve had all season, we were lucky to go into the break with a lead.
“We knew that it wasn’t good enough in the first half. We were lucky to get away with it, and we had to stand up and fight in the second-half.”
Van Gaal meets the club’s co-chairmen Joel and Avram Glazer today to discuss plans for the summer.
With United 12 points behind Premier League leaders Chelsea, the FA Cup looks to be the only chance the Dutchman has of ending his first season at Old Trafford with a trophy.
As such, United cannot afford another repeat performance on Monday when they travel to League One side Preston in the fifth round.
“We need to make sure that we come out of the blocks quickly against Preston,” Smalling added.
“There is (more to come from us). But 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.”
Phil Jones hobbled off with a knee injury on Wednesday to worry Van Gaal. Jones is set for another lengthy layoff but the good news for United is the head injury Daley Blind suffered in the first half was only minor and he looks set to feature against Preston.
United yesterday reported revenues down 14% for the second quarter of their financial year as a result of being absent from the Champions League.
United’s revenues for the three months ending December 31 were £105.7 million (€142m) compared to £122.9m (€165.7m) a year ago — though not as great a drop as some analysts had predicted due to a reduction in the club’s costs and a rise in commercial income.
Woodward also acknowledged the news of a new £5.136bn TV deal for the Premier League had led to calls to clubs to show restraint in spending the windfall on wages and use the extra money for grassroots football and to make it cheaper for fans.
He added: “We have kept season ticket prices flat for five of the last six years and our prices are fairly priced compared to the market.”
The biggest impact on United’s finances has been the lack of broadcast income from European football, down 46.9% to £28.4m.
The wage bill has dropped £2.9m over the three months as the squad size was reduced compared to last season.