"I think we have got something special in this team," the Scot told Skyports.com
Carlos Tevez scored the winner for United in absence of Cristiano Ronaldo, Wayne Rooney and Paul Scholes who were rested by Ferguson.
"I think we had our moments in the first 25 minutes, their goal-keeper has had four fantastic saves," Ferguson told Skysports.com.
Daniele De Rossi missed a penalty in the 28th minute after a tackle from Wes Brown on Mancini that could have twisted the game.
"Then they missed a penalty kick - that would have made a big difference to the game, I think, would have made it a bit more nervous for us," he added.
"But it's a European quarter-final and Roma are second top in the Italian league, you expect them to produce something in the game and I think they grew in confidence a bit."
The Red Devils will face Barcelona in the Champions League semi-finals.