"I think we showed tiredness tonight because the gamble I took about playing on Sunday and then Tuesday was not the best preparation for a European tie," conceded Ferguson.
"But faced with the situation we were in, coming back after internationals, I think I took the best option for us in the hope that a European night would bring the adrenaline up and the occasion would help them."
The European champions now face a tough task if they are to retain their crown because Porto have never lost a home match to a British team in the Champions League.
"It's a difficult job now," stated Ferguson. "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."
And there was further bad news for Ferguson as defender Jonny Evans will miss the Premier League match against Sunderland on Saturday after limping out of the Champions League tie.
The Northern Ireland international was replaced by Gary Neville after 72 minutes and joins fellow central defender Rio Ferdinand on the sidelines.
"You can't say it was even a great manufactured goal," said Ferguson of Mariano's late strike.
"Losing Jonny Evans was a blow because I don't think we'd have lost it with him in.
"Rio was nowhere near being fit for this match, so it is difficult to see him being fit for Saturday.
"Jonny should be OK in a week but he is a 21-year-old lad and this is what happens when you play two big games in three days."
It was a joy to see my team play like that
Porto boss Jesualdo Ferreira
Ferguson traces United's recent defensive problems back to the end of their incredible run of Premier League clean sheets.
United set a new record of 14 Premier League matches without conceding a goal before Roque Santa Cruz scored for Blackburn at Old Trafford on 21 February.
"We were doing very well and not losing any goals because we had a record to defend," commented Ferguson.
"Once we lost that first goal against Blackburn then that record had gone and since then we've lost really bad goals thereafter. What we hope now is to get a settled back four in, not change it and get back to normal."
United started poorly against Porto and Cristian Rodriguez put the visitors in front after only four minutes.
Wayne Rooney capitalised on a terrible backpass from Bruno Alves to equalise before the break and substitute Tevez put United ahead with five minutes of normal time left.
Porto boss Jesualdo Ferreira felt that his team fully deserved to leave Old Trafford with at least a draw.
"In the first half we were the team that attacked more and had more corners. In the second half we were able to keep out United's attacks," stated Ferreira.
"We had a good reaction to United's second goal. It is a joy to see my team play like this."
And he said that the result proved his team have the belief to go and win the tie.
"Much more than the result, the players showed a great confidence and optimism," he added.
"We never forgot we were playing the world and European champions but we tried to make it difficult for them."
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