However, the visitors equalised with one minute remaining and will be considered favourites going into the second leg with the advantage of scoring two away goals.
"Given the importance of the game it was a terrible goal to lose and we've only ourselves to blame," Fergie said after the 2-2 draw.
"I think it exemplifies the fragile nature of our defence at the moment.
"It's a difficult job now and it's not made any better when they tell you that no British team has won in Porto, so we have a job to do.
"We have to think that the last goal we've lost tonight is one we couldn't afford to lose, it was a terrible goal to lose.
"We were 2-1 up, seeing the game out, it would have been a normal situation for Manchester United."