Jose Mourinho's appointment and some marquee signings helped bring a feelgood factor back to Old Trafford after the prosaic reign of Louis van Gaal.
United enjoyed a dream start to the campaign as they won the Community Shield and their first three Premier League matches, only for that run to come to a shuddering halt on Saturday.
The 2-1 home defeat to Pep Guardiola's Manchester City was compounded as Feyenoord eked out a late 1-0 win in Thursday's Europa League opener.
Stand-in captain Smalling admitted "we didn't play up to our standards" in Holland and that United can ill-afford another loss when they return to Premier League action at Watford on Sunday.
"I think if anything, that's a good thing," the centre-back said of the quick turnaround.
"We don't have time to dwell and we have a few days to recover.
"We make sure that we stand up and be counted on Sunday because we've got a big result to get.
"I think (the confidence) does take a little knock, but I think we can't forget some of the chances we created.
"I think on Sunday character...needs to shine through because it's unacceptable to lose three games in a row."
United struggled for inspiration and coherence at De Kuip, where unfancied Feyenoord grabbed three points from the Group A clash.
Jose Mourinho was frustrated by the result and the fact Nicolai Jorgensen was not called offside in the build-up to Tonny Vilhena's later winner - something that irked Smalling, as did the overall display.
"I've not seen it myself but it's unfortunate if that is the case," he said when MUTV mentioned the missed offside call.
"But I think as much as they didn't create too many chances, I think on our part we didn't create enough chances either to really go and win that game.
"In the second half, I think our urgency was there but we need to start that in the first half."
United attacked with numbers in search of an elusive last-gasp leveller as the clock wore down in Rotterdam.
There were a handful of late chances but Smalling knows it was too little too late in a match where they were too often outbattled.
"I think we upped our intensity," he said, having made his first start of the season.
"I think in that first half they won pretty much every second ball and they had aggression and determination far better than ours.
"In the second half, I think we matched that but we need to do that earlier to give ourselves a better platform to win the game."
Source : PA
Source: PA