We did not want to be the clowns – Smalling reveals Mourinho's half-time pep talk
Pep Guardiola's side appeared to be coasting to victory against their neighbours when they led 2-0 at the halfway stage of the contest.
However, knowing a win would see their rivals seal the Premier League title, the visitors rallied after the break to turn the match around and clinch a stunning 3-2 victory.
Paul Pogba scored twice to make it 2-2 before Smalling popped up to be the unlikely hero for Jose Mourinho's men, volleying home a 69th-minute winner to delay City's celebrations.
"At half-time a lot of us were feeling sorry for ourselves. We needed to make sure we played with pride in that second half," the England international told Sky Sports.
"I think the first half was terrible. Apart from the first 15 minutes, then we just dropped off. We didn't play our game and they could have been out of sight.
"He [Mourinho] didn't have to say too much because as players you know that was not good enough.
"The emphasis was that we did not want to be the clowns standing there watching them celebrating winning a trophy at the end.
"You saw us stand up. At 2-0 down away at City, where they’ve been fantastic at home all season, we stepped up and won the game."
Smalling's winner came after he had lost Vincent Kompany at a corner for City's opening goal.
Ilkay Gundogan's turn and finish made it 2-0 after 30 minutes, but Raheem Sterling twice fluffed his lines in front of goal before the break, blazing over on both occasions to let United off the hook.
"When you make a mistake like that, you want an opportunity to make amends. Thankfully, it came my way," added the centre-back.
Passion level: 100
This one's for you, #RedArmy - Manchester is red #MUFC pic.twitter.com/FFS1AmJaIg
— Manchester United (@ManUtd) April 7, 2018