Carlos Tevez's strike five minutes from time looked to have given United an advantage they barely merited as they looked a pale shadow of a side on course to collect an historic haul of five trophies.But as Old Trafford celebrated with a mixture of relief and elation, Porto - United's conquerors in this competition five years ago - delivered what could be the telling blow when substitute Mariano scrambled home an equaliser with a minute remaining.
Cristian Rodriguez gave Porto the lead inside four minutes, his sharp finish coming as a result of the defensive uncertainty that plagued United from start to finish.
And even though Wayne Rooney swiftly levelled after he swooped on Bruno Alves' back-pass, there was to be no escape for United from a lacklustre performance that leaves them in serious danger of relinquishing their Champions League crown in Portugal next week.
