In his first start in over a year, Havertz created Viktor Gyokeres' opener before scoring himsekf in the 3-2 win.
"Unbelievable," said Arteta of Havertz's impact. "For a player that has been out for so long to perform with that clarity, with that energy, with that efficiency, very impressive.
"We know Kai, how much he can give us in different positions, different roles, different contexts. He's a player that makes us much, much better."
Brando Bailey-Joseph and Ife Ibrahim, fresh from signing a first pro deal this week, made their debuts on the night.
Arteta said: "Very good, very happy and I think it's a reward not only for them but everybody that works in the academy in that process to educate, to raise them and to get them to the point where we can use them in a Champions League match and I'm very happy.
"To win eight games in a row in this competition is something very, very difficult to achieve, so big credit to the players for doing that.
"We closed the first part of the Champions League in the manner that we wanted, being first and not having to play another two games and having the home advantage in the second leg and after that, after making 11 changes, I think the team's attitude was spot on from the beginning. We had some very good individual comebacks and performances."