Sagna played an hour in a 2-0 victory for the reserve side against Reading, in his first appearance since breaking his leg against Norwich City in May.
The French international told the club's official website that he is happy with his recovery at this stage.
He said: "It is amazing to be back, I have been waiting for this moment for five months. My target was to come back and play, firstly for the Under-21 team. Everything went well and we won, so I am very happy.
"I have been working really hard. I spent some time in the rehab centre, then I came back and worked really hard with the fitness coach and the physios to be ready to play and finally I have.
"When you have such a big injury once, you take it, but twice is hard. In my mind I was a bit sad, but the next day I was ready for the challenge, which was to come back stronger and I am in a good way today."
The Gunners are also close to having Jack Wilshere back in first-team contention after 14 months on the sidelines, with the England midfielder playing alongside Sagna at Underhill on Monday.