Smalling was on the scoresheet in victory at Hull City on Boxing Day.
"We hope they (other teams) are getting nervous," said Smalling. "We're starting to get on a good run.
"If people are starting to look over their shoulders, we're doing a good job.
"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.
"We're in a rich vein of form. It's one of those seasons where there will be a few ups and downs and some teams have yet to have their downs. We have had our blip and can't really afford any more."