The two Manchester clubs, eliminated from the Champions League in December, are both on course to reach the last 16 of Europe's second-tier competition this week. City are the first in action as they look to protect a 2-1 lead over Porto at the Etihad Stadium tonight while United, 2-0 ahead, face Ajax at Old Trafford tomorrow.
Having been kept apart in the quarter-final draw, the possibility remains they could meet in the final in Bucharest in May, but Nasri said: "It would be a good derby in the final but we are not there. We have the game against Porto first and then after we will see."
He added: "United are a contender, for sure, but there is not just Man City or Man Utd, there are plenty of teams."
City underlined their determination to take the Europa League seriously last week as they fought back from a goal down at the Estadio do Dragao to claim a first-leg lead.
It has been suggested the competition could be a distraction as City, currently two points clear at the top, fight for the Barclays Premier League crown but manager Roberto Mancini wants both titles.
Mancini said: "For us, every game is serious. We want to win all the games. Sometimes that is possible, sometimes not, but for us it is a serious competition.
"If we win the Europa League it will be an important moment for the club, but I don't think it will be easy. In this competition there are top squads but we want to go to the final. It is an important target for us."
Last week's victory was overshadowed by alleged monkey chanting at City players, chiefly Mario Balotelli, by Porto fans.
European governing body UEFA yesterday confirmed they had opened disciplinary proceedings and would deal with the matter at a meeting of their control and disciplinary body on March 29.