Sir Alex Ferguson admitted Manchester United could not afford to gift FC Porto their late equaliser at Old Trafford.
Barring an even bigger score draw, the 2-2 scoreline means the Champions League holders must win or go out at the quarter-final stage.
It did look as if United would collect a precious lead from the first-leg tie despite being outplayed during the opening period.
But Carlos Tevez's 85th-minute effort was quickly cancelled out by Mariano, to leave Ferguson to reflect on what might have been.
"You have to think the last goal was one we could not afford to lose," said the United boss.
"It was a terrible goal to give away when you are 2-1 up and seeing the game out.
"It is a difficult job now, not made any easier when they tell you no British team has won in Porto.
"But we only have ourselves to blame. We just have to win. We have been first at everything at this club. Let's hope we can be first again."
After their last-gasp heroics against Aston Villa on Sunday, United appeared to be caught off guard by Porto's flying start.
What was once the most watertight defence in Europe failed to come to terms with the threat Porto posed and the visitors could easily have scored more than the single first-half goal they did manage through Cristian Rodriguez.
United got on top in the second-half and when Tevez struck at the near post, it seemed victory would be secured.
"At that point in the game you just think 'excellent' because we had not played particularly well," said Ferguson.
Instead, Porto were allowed to equalise once more and news that Jonny Evans will join Rio Ferdinand on the sidelines this weekend when United head to Sunderland only made a bad night even worse.
"Rio was nowhere near being fit tonight, so it is difficult to see him being fit for Saturday," said Ferguson.
"Jonny should be okay 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."