Asked if Rooney appeared down when he was told, Smalling replied: “No.”
The England centre-back added: “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.”
Saturday was the first time Mourinho had dropped Rooney since he took charge of the Red Devils in the summer.
Smalling, 26, said the England captain acted in the same manner whether he was in the team or on the bench.
“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’s why he is England’s main man and our main man,” he said.
Rooney needs just four goals to replace Sir Bobby Charlton as United’s record goalscorer – and Smalling does not think it will be long before he returns.
The former Fulham player said: “He’s a very experienced guy and he’s played that many games that I think it’ll only be a matter of time before he’s back in there and firing again because he’s quality.”
Smalling headed home the opening goal in United’s victory over Leicester, with Juan Mata, Marcus Rashford and Paul Pogba also scoring.
Asked about Rooney after the game, Mourinho said: “He’s my man, I trust him completely. He’s as happy as I am at this moment.
“He’s a big player for me, for United, a big player for this country.”
Analysis: Is Rooney surplus to requirements?
BBC Sport’s Simon Stone
“There is no great disconnect between Wayne Rooney and club or manager, as there was in 2013, when Rooney sat in an executive box and watched Sir Alex Ferguson’s final Old Trafford game as Manchester United manager behind glass.
“Rooney politely applauded back when he was sent out to warm up for the first time, and the home fans’ reception was enthusiastic when he ran on to the field after replacing Rashford near the end.
“It won’t stop the chatter around Rooney though. His United future is on the line.
“What we don’t know yet is whether England’s captain is surplus to requirements permanently.”
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