The Red Devils were superb in the absence of Rooney, who made a late cameo as a substitute, scoring four first-half goals to leave Leicester shell-shocked.
Mourinho later offered his backing to Rooney, though, and Smalling – who acted as stand-in captain – says the 30-year-old was not affected by the decision and is sure his club and England team-mate will come back strong.
“Wayne did not appear down. He was the same before the game when we were all getting ready. He is often one of the most vocal and he was the same,” Smalling told reporters.
“Whatever the situation is, whatever game, whether he is on the bench or playing or whatever, he is always that same type of character and that is why he is England’s main man and our main man.
“He is a very experienced guy and he has played that many games that I think it will only be a matter of time before he is back in there and firing again because he’s quality.”