Alan's double within 20 minutes stunned the home crowd and ensured the men from Manchester had fallen behind in eight of the 12 fixtures this term. However, the two-goal lead was eventually overturned on a European night containing much excitement for the 73,195 fans in attendance.
Javier Hernandez pulled one back with a header and nodded the 75th-minute winner after Jonny Evans poked in the equaliser from a corner. The Mexican striker looked back to his clinical best which could be bad news for Sunday's opponents Chelsea considering his previous form against the Londoners.
The Reds made a disastrous start by conceding a corner inside the first minute and falling behind moments later. Hugo Viana's cross did not appear to carry huge conviction as he swung it into the danger zone but Alan beat Alexander Büttner to steer an unstoppable header wide of David De Gea.
Clearly unfazed by the occasion, Jose Peseiro's side passed the ball crisply and with confidence during the opening stages as United, with captain Wayne Rooney in a deep role, struggled to mount any pressure on Beto's goal. Ruben Micael's warning shot wide was not heeded and Eder tricked his way past centre-back Michael Carrick to tee up Alan for a well-taken second.
Sir Alex's side found themselves in the familiar position this term of needing to stage a comeback and, within five minutes, the deficit had been halved. Robin van Persie was up-ended after showing neat skill on the left but referee Milorad Mazic allowed the advantage with Shinji Kagawa in possession. The Japan international lofted across for Hernandez to stretch and head