Harald Brattbakk has warned Celtic that Rosenborg have not been at their best so far this season, indicating there is much room for the Norwegian giants to improve.
Brattbakk's two former clubs met at Celtic Park for the first leg of their Champions League third qualifying round tie last week and played out a 0-0 draw which means the contest is very much in the balance.
Celtic are still widely tipped as being likely to have too much for Rosenborg in the return leg in Norway on Wednesday, but Brattbakk feels his countrymen have not yet hit their best.
The former striker explained that many were surprised Rosenborg were able to draw at Celtic, given their lacklustre domestic form.
"They haven't actually played as well as people would expect", he told BBC Radio Scotland's Sportsound programme.
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"It surprised a lot of people they came away with such a good result, but also the way they came away with that good result.
"It wasn't a fluke, it was hard work", Brattbakk added.
Rosenborg may expect to hold a fitness advantage over Celtic as they are already well into their Norwegian league campaign.
Currently they top the table with 35 points from 17 games, having lost twice.
