The Gunners will come face-to-face with Oxlade-Chamberlain for the first time since his £40million summer move to Liverpool when Jurgen Klopp's men visit the Emirates Stadium on Friday night.
The England international decided to call time on his six years in north London having never established himself in Wenger's first-team plans.
The Gunners had wanted to keep the 24-year-old, who still had a year left to run on his contract, but it is understood Oxlade-Chamblerain was keen to move on in the hope of joining a team who could offer him regular game time.
The midfielder has yet to achieve his goal of more first-team football at Anfield, starting just five Premier League games this season, and could well be on the bench again on Friday.
Wenger has now admitted he had expected the former Southampton youngster to stay and keep working on trying to establish himself in the Arsenal team, especially given he was not guaranteed to play more whoever he joined.
"Is he sure of a place there?" Wenger said at a pre-match news conference. "That's the answer. You are sure nowhere. In a big club, you have big competition for every single place. That's absolutely normal.
"Look, he had one year to go. At the start of the season we had four players with one year to go [Oxlade-Chamberlain, Alexis Sanchez, Mesut Ozil and Jack Wilshere]. I wanted him to extend his contract, yes. He decided to go. We respect that but at the end of the day, I decided to sell him.
"I believe that he made this decision and we had to find a compromise to get the best possible transfer from it. That's part of it. I personally think he will be an important player for England and that he will continue to develop.
"What do you want - to be a big player and not fight? What do you want on top of that, sitting in a rocking chair saying 'I'm a big player - I don't need to fight'? It does not work like that. Every player can do that and he does it so overall I am convinced it is part of being a top player."