Havertz converted an 89th minute penalty won by Noni Madueke to cancel out Robert Andrich's opener for Bayer in the first-leg of their round 16 tie in Wednesday.
Arteta said of Havertz's spotkick: "Very important. The game had different phases, some in the first half when we were very dominant and we had a big chance with Gabi to make it 1-0 and when you miss those chances, obviously the game can change.
"Then after that, we didn't finish enough actions, every time we got in and around the box, we gave the ball away and that allowed certain counter-attacks and that was very unlike us. Without really considering anything, we needed to show certain things.
"We discussed that at half-time and we expected to start very fast, especially because we knew certain routines that they have on kick-offs, because they had three at the weekend and still, we got caught. We weren't ready enough; we conceded a chance on the header and after that we conceded the goal.
"So, when you give that hope and advantage to the opponent at that level, they're going to take it and then we had to really dig in because emotionally it becomes a very different challenge. We had 10-15 minutes that we didn't really have enough threat and enough understanding of how we had to attack that block, but in the last 20 minutes, 25, with the changes I think it was much better. In the end, we found a way to score the goal and draw the game."
On Havertz's coolness in taking the penalty, Arteta added: "How composed he took it and yes, football is a funny game, and it brings special stories. Him coming back here after such a long time, being part of this club, to come here and score such an important goal, I think it's a big moment."