The Scot admits it was tough letting the young defender know he was to drop out of the team in place of Rio Ferdinand for the Champions League meeting at Stamford Bridge
After deputising for the former Leeds United player, Smalling has put in a string of solid performances that have impressed boss Ferguson.
The Scot insists that while the young centre-back understood the decision to bring back Ferdinand for the game that United edged 1-0, he still found it difficult to break the news to the former Fulham man.
"It was terrible for me to explain to Chris Smalling that wasn't playing given the way he has performed for us this season," said Ferguson.
"He understood. He is part of the squad and he won't be forgotten. But it's Rio's experience isn't it? He has such composure. He talks to players.
"When Antonio Valencia went to full-back he was talking all the time. He was phenomenal."
Ferdinand had been out for two months prior to the victory at Stamford Bridge but lasted the full 90 minutes of the quarter-final first leg and Ferguson admits that the England man was desperate to play.
"I was waiting for the moment where he might tire a little bit but there was no sign of it," he added.
"He had done 10 days' physical work before he started football training. People might think it was a gamble.
"My only gamble was if he wasn't really up for it. Whether we had to talk him into it. That wasn't the case. He came to me. He said I am ready. It was great news."