A 2-1 defeat which also saw the Royal Blues lose Benedikt Howedes and Christian Fuchs to red cards leaves them with just a single point from their first three games of the season and the pressure is already rising on Keller.
However, he says the result did not reflect his side's performance and, like in the 1-1 draw against PAOK FC in midweek, he saw more positives than negatives.
"If we manage to combine the second half against Hannover with the first half against PAOK, then I will be happy," he said.
"The way we played after the break was astonishing. With the big chance to make it 2-2, it was almost harder not to score.
"Given the second half, we would have deserved a draw, but ifs and buts don't count for anything when you go home without any points."
Schalke travel to Greece on Tuesday for a pivotal game in terms of the rest of their season.
An early exit from the Champions League would only compound their woes and Keller hopes they will soon be able to get out of their current slump.
"It was not easy for us to take to the field with our heads held high and we made many mistakes in the first 10 to 12 minutes, and then we got the red card," he said on his club's website.
"After that, it's incredibly difficult to get a foothold when you have the PAOK game in your legs, you only have 10 men and it's such a hot day."