David Moyes’ men travel to Carrow Road to face Norwich City after recording a 3-2 victory over Hull City on Boxing Day.
United are currently seventh in the table, eight points behind leaders Arsenal and Smalling has urged his team-mates to maintain their momentum.
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"We have to take each game as it comes and we have a long way to go yet.
"It's about keeping this momentum going. Right now we're in a rich vein of form and we need to get back to the training ground, recover and then go to Norwich.
"It’s one of those seasons where there are going to be a few ups and downs and some teams have yet to have their downs.
"We have had our blip and we have to make sure we don't have any more because we can't really afford any more.
"Even when we are flying, people may doubt us. We're here to prove those doubters wrong and to give all those fans who travelled to Hull something to go back happy with.
"Despite our start, I don't think you can ever write off United because of what they have done over the years.
“We knew our time would come and hopefully this is the start of a really good second half to the season where we can put the pressure on and move up the table."