Korb conceded a penalty, duly converted by Jefferson Farfan on 17 minutes, but referee Felix Zwayer only showed the Mönchengladbach defender a yellow card.
Keller said a red card would have been in order and instead, on the stroke of half-time, one was shown to Schalke's Benedikt Höwedes.
"In the first half, we played the way we had agreed before the game: stay deep and tight, chase the ball down and get forward quickly after winning the ball," Keller said.
"We did that well for large periods of the match and going ahead 1-0 played into our hands.
"I also think Julian Korb should have been shown a different card, as then we would have been able to keep pressing forward with a one-man advantage.
"Instead, the opposite happened.
"My team showed impressive commitment after half-time and pushed to score an equaliser against the team with the strongest home record in the league.
"A second goal and a point for us would certainly have been well deserved."
Mönchengladbach coach Lucien Favre also said the dismissal of Höwedes, for picking up two cautions in 10 minutes, was unwarranted.
"We turned the match around after going 1-0 down. Raffael scored a super goal and Max Kruse converted his penalty ruthlessly. The second half was very difficult for us," Favre said.
"We overplayed too often, but it's not easy to play against a 4-4-1 formation.
"We had chances to score a third goal and wrap things up but failed to take them. It was close after that.
"Schalke are strong in attack and always dangerous.
"We were lucky that (goalkeeper) Marc-Andre ter Stegen hung onto the ball in the final minutes; at first I thought it was a goal.
"Ultimately, we've got to be pleased with the win. I think Benedikt Höwedes' red card was harsh. Rules are rules, perhaps, but I don't think it's right."
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