Real Madrid midfielder Xabi sees Man Utd as favourites
Real go to United with the aggregate scores locked at 1-1.
"[Manchester United] are a squad that are at home and, in the circumstances ahead of the match with the tie so open, they're going to have lots of support from their fans," Alonso said.
"It's going to be a challenge and the side that scores first is going to have a good chance of moving on. Old Trafford is a great stadium and it's the kind of match we all like to play."