Jose Mourinho's Red Devils enter the international break second in the Premier League but eight points behind leaders Manchester City after Sunday's 1-0 loss at Chelsea.
There has never been a larger gap between the top two after 11 matches in Premier League history and Pep Guardiola's swashbuckling side are as short as 1-9 with some bookmakers to win the title.
United have to wait a fortnight to close the gap and Smalling has warned Newcastle that they face a difficult afternoon when they return from the international break.
"It is unfortunate (to have to wait)," the centre-back said
"I think we'll have to regroup and make sure that - our next game is at Old Trafford - we put on a performance to rectify this.
"It is about us getting back to Old Trafford for that game and putting on a performance that the fans can be proud of."
Having been overlooked by England for their upcoming friendlies, Smalling has been given a week off by Mourinho for some rest and recuperation.
The 27-year-old may well spend that period stewing on Gareth Southgate's comments about his ability to play from the back, or Chelsea's avoidable winner on Sunday.
Alvaro Morata - a summer target for United before Romelu Lukaku arrived - got between Smalling and Eric Bailly to head home Cesar Azpilicueta's cross.
"I think it was two teams there who can defend and who have got very good attacking players," he told MUTV.
"I think it was always going to be a close game and we just fell asleep on their one big chance, unfortunately.
"It was disappointing from us because we'd watched a lot of their videos and we knew about Azpilicueta coming up and putting in those crosses.
"So for us to know what their game plan was and to fall asleep is very disappointing."
That lapse in concentration compounded a lack of cutting edge in front of goal
Marcus Rashford and Marouane Fellaini had late attempts to snatch victory, but it was the failure to capitalise on early momentum that Smalling felt was key.
"I think we did put in so much," he said.
"I think our first half where we controlled large parts of that game and we put them on that back foot, and we just needed to take one of those chances in that first half when you've got that momentum and you can keep squeezing it on them.
"But we just couldn't take one of those chances.
"I thought those last 20 minutes we did put on some pressure and we were really on that front foot, and I thought a chance might come because I thought we deserved a goal."
Put to Smalling that he did not start the second half as well as the first, he said: "I'd agree
"We had that momentum, you could sense the crowd was on them in that first half.
"I think we didn't come out in that second half with that same pressure."
Source : PA
Source: PA