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."
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."