Saints are underdogs in their Europa League second qualifying round tie against the Norwegian side, who are top of their domestic league after 16 matches.
But Mannus has fought against the odds in Europe throughout his career with Linfield, Shamrock Rovers and now St Johnstone.
The Northern Ireland international helped Rovers beat Israeli side Bnei Yehuda and set up a glamour tie in the 2010 competition, before joining Saints ahead of the Irish side's unlikely charge to the Europa League group stages.
The 31-year-old told Press Association Sport: "I've had some good results over the years. Probably the best was with Shamrock Rovers when we played an Israeli team and drew 1-1 at home. The draw was made before the second leg and the winners came out against Juventus.
"We weren't given much chance to go through because they were a good team and the weather was very hot but we won 1-0. They were all over us but we ended up getting Juventus in the next round.
"It was just a battle for us, we battled through."
Mannus expects to have to go through a similar experience if Saints are to progress.
"It's difficult these kind of games because Rosenborg are already into their season," he said.
"The thing I found with Shamrock, whenever we went into European games we were halfway through our season so you know what your starting 11 is going to be, you know how you will set up and everyone is comfortable with the way you are playing and you are in your rhythm.
"It makes it a bit more difficult when you haven't had a competitive game yet but anything can happen. We are still a good team with good players and hopefully we can do a good job."