Substitute Federico Macheda scored the winning goal to send United back to the top of the Premier League with a 2-1 victory over the Black Cats.
"When they got the equaliser, maybe a little bit against the run of play, nonetheless it puts you under pressure to make decisions about your changes. Fortunately, I made the right one," Fergie said after the match.
"He [Macheda] knows the role we want him to play. We want him to penetrate in behind them.
"He has got good movement, he has got quickness in his movement and he showed that when he came on.
"He wasn't fazed by anything. Right away, he got involved in the game and he had a good 20 minutes for us."
And on the result, Fergie added: "It's a boost, of course. The important thing is we have seven games left now and at this point, it is about cutting each game off as they come along.
"This was an important away victory for us. It's a difficult place to come to against a very committed Sunderland team and we can be pleased with that."