كتب : Omar AbdelAziz
The defending champions were punished by a late strike from Mariano Gonzalez at Old Trafford, where they looked vulnerable for long spells of the first-leg tie.
Ferguson lamented the soft goal as he declared that Jonny Evans' earlier injury had a negative influence on his side's backline, who already missed Rio Ferdinand.
"It's a difficult job now, I agree. The last goal we lost was one you can't afford to lose. It was a terrible goal to concede. At 2-1 we should have seen the game out, we've only ourselves to blame," he said at the post-match conference.
"It wasn't a particularly well manufactured goal. It exemplified the fragile nature of our defense at moment. I don't think we'd have lost the goal had we not lost Evans. We'd moved (John) O'Shea into centre-back, (Gary) Neville got sucked in and it was a bad goal."
After a run of 14 successive clean sheets in the Premiership, United leaked in 11 goals in the last five matches, following Roque Santa Cruz's strike for Blackburn in February.
Ferguson believes that losing the record two months ago is still haunting his team.
"We'd been doing well when we were not conceding goals because of the record, we had something to protect. Since we lost the record against Blackburn we've lost a lot of bad goals," he said.
Good Result
At the other end, Porto boss Jesualdo Ferreira was left delighted with the performance of his team against the European titans.
"I would like to congratulate my players because they deserved what they achieved. We never forgot we were playing against the reigning world and European champions but we tried to create problems for United and we managed to do that," he said.
"It's a good result to take to the Dragao but the tie's not settled yet. They'll know that they need to win or draw by more than two goals to go through. Whatever happens, nothing can take away what we did today."
Ferreira singled out praise for young duo Aly Cissokho and Fernando, who managed to contain the danger of United ace Cristiano Ronaldo throughout the game.
"It's great to see two players like Fernando and [Aly] Cissokho, who are very young and pretty much still boys, showing such ambition, ability, concentration and maturity. It means a lot to work with a team like this," he added.