Wenger rates the versatile Nordtveit highly but he believes the 20-year-old needs regular football to continue his progress.
"He was not even practising every day with the first team, so he needs to play," the manager told arsenal.com.
"He is a fantastic character, with great quality. He played last year because we gave him to Nurnberg [on loan], he came back and didn't play and he needs to play now. That's why we let him go.
"Yes [I had high hopes for him], but we have a number of centre backs and full backs so he didn't get the games. I thought in December that it was the right moment for him to go and play."