The 31-year-old has been ensuring United don't leak goals at the back, while his attacking team-mates have been ruthless at the other end of the pitch.
Jose Mourinho's men put on another classy display as they made light work of CSKA Moscow in the Champions League earlier this week.
Lukaku and Martial both netted in Russia, and Smalling believes the pair are giving their opponents nightmares.
"He’s just a beast,” Smalling said of Lukaku on MUTV.
“You can play it into him and he can hold it up, people can run off him, and he’s just a nightmare to play against.
“Right from pre-season, Anthony’s been leading the way in training,” he added.
“He’s terrifying one-v-one, as we’ve seen, I’m glad he’s on my team.”
The Red Devils take on Crystal Palace today, as they look to pile more misery on Roy Hodgson's struggling side.
And Smalling says United are heading into the clash on the back of their best performance of the season.
“I think that’s as dominant as we have been,” Smalling told BT Sport after the CSKA clash.
“Scoring those goals in the first half, it was probably the best first half we have played all season."
United currently sit level on points with rivals Manchester City at the top of the Premier League table.
