Nasri moved to Manchester City for £24 million in August after refusing to extend an Arsenal contract which was due to expire in 2012. Only weeks before the transfer, Wenger said he was confident that the France international would stay with the London side.
However, it appears his optimism was little more than a smokescreen designed to extract a larger transfer fee from City. "When we didn't reach an agreement [over a new contract] I prepared myself to face the possibility he may leave. I knew in May," Wenger said.
Nasri joined Arsenal for £11m from Marseille in 2008 and, particularly across his final season at the club, emerged as one of the finest midfielders in the Premier League.
Ahead of City's trip to Arsenal in the Carling Cup quarter-finals on Tuesday, Wenger has urged the Gunners fans to remember the good Nasri did for the club - and not the manner of his exit.
"I hope Samir gets a good reception," Wenger said. "I want every player who played for us to be respected when they come back. Samir improved tremendously with us. He has developed fantastically well from a non-scoring player to a guy who can make a decision."
Wenger has warned Manchester City their biggest tests are yet to come.
"You have to go through the whole winter when you play in the Champions League - we did that," he said. "In the FA Cup as well, the semi-finals of the Champions League - to do that together is very difficult.
"It [an unbeaten league campaign] looks unlikely, but you never know. You can never really doubt. However, in the modern game you need a bit of luck - watching the game against Liverpool they could have been beaten.
"Overall hopefully for us not, because we want to get as close as possible to them and it is important to beat them.
"Financially it is impossible to compete with them. On the football pitch it is possible. Our squad has more experience and I am confident we can show that. We are still a young team. The success of this club is based on the development of young players and we still want to show that.''
Striker Mario Balotelli and midfielder Gareth Barry are suspended, but may have been left out anyway after featuring in the weekend draw at Liverpool. Former England midfielder Owen Hargreaves, fit after a recent calf injury, and Stefan Savic are among the more high-profile players who could be involved.
Wenger has confirmed he will give some of his fringe players a chance to impress. Alex Oxlade-Chamberlain, Emmanuel Frimpong, Yossi Benayoun and South Korea captain Park Chu-Young are all in contention as some of the Gunners' key men will be given a break.
Midfielder Abou Diaby is a doubt after picking up a muscular injury in Saturday's 1-1 draw with Fulham.