United are hot favourites to win in Oslo, but Sir Bobby does not intend to take any risks with his team selection.
"I don't think I should muck about with the team because we don't know who we are playing,” he told the Sunday Sun.
“They don't play against any opposition we can judge them against.
"We could go there, play around with the team, lose 2-0 and get knocked out.
"We should find out in the first match how good they are and we've got the second leg to change things if we can."
Valerenga coach Kjetil Rekdal meanwhile has promised to make life as difficult as possible for United on Thursday.
"I can guarantee you that the 11 players who start against Newcastle won't be scared of the opposition or feel the tension of the moment," he said.
"We will demand respect and we won’t be made a fool of.
"It won't be easy, I know, but we don't intend to make it easy for Newcastle either. England is a great football nation, but they have lost to Norwegian teams before.
"This is all about preparations, tactics, effort and a little bit of luck.”