The 20-year-old has now helped Schalke win both of their Champions League games to top Group E, but believes that with a double-header against Chelsea next up the Germans need to remain grounded.
He told the Bundesliga's official website: "The ball fell rather kindly for me. You probably hit three out of ten balls like that in training.
"This time it came off. Of course it was a bit lucky, but it's the small things that decide games in the Champions League. I can definitely do better.
"It's great to be top of the group with two wins and no goals against, but we shouldn't read too much into it. There's still a long way to go. In Chelsea, we've got to play possibly one of the strongest teams in the competition - twice."
Keller's men will now be hoping to replicate their European form in the Bundesliga, where they have won just two of their seven matches so far this season.