Metzelder, 30, conceded United are overwhelming favourites to go through, but insisted Schalke love being the outsiders.
He said: "Manchester United have had so much experience in the Champions League over the last 20 years.
"Individually they are so very strong so they're big favourites."
Schalke striker Raul aims to knock United out of Europe for the third time after two victories with Real Madrid.
Metzelder added: "He's a big weapon because he's always successful. Wherever he plays he wins trophies and we have to take care of him."