Zlatan Ibrahimovic and Paul Pogba both returned to the first-team fold in United's 4-1 win over Newcastle United last Saturday, adding to the attacking firepower of Romelu Lukaku, Anthony Martial and Marcus Rashford.
And Smalling believes even he would be cautious if he came up against such a side.
"I think it would strike fear into myself if I was playing against them, and it strikes fear into most teams," Smalling told reporters before United's Champions League clash with Basel.
"Obviously we've got Paul [Pogba] back too, who's a very attacking midfielder, and along with Ibra and all those other names it just gives us so many options and no matter what game it is, no matter what competition it is, we've got a front three or four who can cause the opposition problems.
"So if we keep it tight at the back, more often that not, we're going to win the game."