Jose Mourinho's side find themselves falling under mounting pressure just two games into the 2018-19 campaign.
Smalling admits as much, telling the club’s official website: “We have to regroup and we’ve had a good amount of days on the training pitch.
“It’s about putting that hard work in, then standing up and showing some character. We need to come out fighting.”
United have seen the likes of Liverpool and Chelsea extend their faultless starts to the season to three games, as they set the early pace in the Premier League, with Smalling eager to get going and right the wrongs of the Red Devils’ last outing.
He added: “To be honest, I’d rather have had the game on Saturday because you just want to bounce back after a defeat.
“It’s a long weekend but, at the same time, we've had a long time to prepare for the game and we’ve tried to take advantage of those days.
“This is a good game for us to have. When you lose a game, the next one is more important than ever.
“The fact that it’s a big game at Old Trafford, that’s when the big players rise to the occasion. We’re determined to be ready.”
Smalling is among those hoping to be drafted into United’s starting XI by Mourinho, with centre-half rivals Eric Bailly and Victor Lindelof having endured an error-strewn afternoon against Brighton.