The fullback was sent off during the 8-2 humiliation.
Sagna told Arsenal Player: "I don't need to say anything to him, he gives his best day by day. The whole team struggled, not only him. It was difficult to defend as a team and Manchester United played really well.
"They gave a hard job to everyone, not him in particular. I think he is a good player. He is very talented and is is still learning - he is still only 19 years old. He still has time to improve and he will for sure."