The youngster is a firm favourite of manager Antonio Conte, and his impressive performances in central defence are being noticed by ex-players too.
Gallas had some advice for Christensen, along with a lot of praise.
"He is still young and what he does is unbelievable," Gallas told Goal.
"He is the main central defender for Chelsea and everybody makes mistakes. The best central defenders make mistakes. So for me, I am not worried for Christensen because when you see him playing, he is not scared because of his age.
"He takes his responsibility. He knows how to speak to his team-mates. For the next game, you have to be sure that he won't make the same mistake. So for me, for the future, he will be a great, great defender.
"For now, we need to know if he can only play in that system with three centre backs, or if he can play with four defenders, alongside one team-mate. For the moment, in that system, he looks great. I don't know if he will look great if you play with four defenders.
"It shows that the academy of Chelsea is really good because now everybody knows you have to spend a lot of money to bring in players. Sometimes money doesn't make you look great, or doesn't allow you to play very well.
"He is still young and we don't have to put pressure on his shoulders but I had the opportunity to meet him at Cobham. When I say meet him, I just mean to say hello to him, but when I said hello, I heard from the boys that he is very mature in his head.
"I know he will have a good career. If he listens to the more experienced players, he can be one of the best defenders. He has to improve by maybe speaking a little bit more on the pitch, to be a leader in his defensive line. That is very important in football, the communication.
"When I watch Chelsea, I don't see him speaking a lot with his team-mates, maybe because he is focused on what he has to do. Some players like to do this, just to be focused on their job, but slowly, slowly he has to speak to his defensive line and midfield. That's what John Terry used to do."