"My job also is to challenge them for the best of the team.
"[It is a] quality team, quality players but they have to believe a little bit more in themselves. Tactical flexibility - we have players who can play in different positions. For me this is important to have players who I want to play in two positions minimum.
"We play against a style of play, we don't play against the name of the team. Roma, the way they play is not the same as Hearts. We know what we have in front of us. We know what we want to do."
Nancy's whirlwind start at Celtic continues next Sunday with the Premier Sports Cup final against St Mirren and his first meeting with city rivals Rangers is scheduled for 3 January, after four more league games.
The former Columbus Crew and CF Montreal head coach was asked about the January transfer window and hinted there was a "profile that we need".
"Each organisation, the idea is to improve," he explained.
"I'm going to have to assess the team. I know them as a team because I've watched many games. Now it's about knowing them as a person. After that we'll see what is the best for the team.
"I like to take my time. I know that I don't have a lot of time.
"It's not a secret. I want to play in a certain way. Proactive, try to put out on the opposition, try to attack the ball as soon as possible and after that also be disgusting to play against when we defend because we're going to have moments when we've got to suffer."