The North Rhine-Westphalia-based club have conceded 26 Bundesliga goals in just 14 games this season and Keller admitted those defensive frailties are a concern.
Keller said a high-scoring result is not what he is chasing at their Veltins-Arena home in Gelsenkirchen on Tuesday, and said improvement was still necessary - despite Schalke having chalked up 202 consecutive minutes without concession in all competitions.
"It's not fun to play 3-3 or 4-4," Keller said.
"It's a knock-out game tomorrow so the most important thing is that we qualify regardless of what the final result will be.
"I want the team to be as focused defensively in this game as they were on Saturday (against Stuttgart).
"That needs to be our goal, to be stable in the back and concede less.
"We have a lot of quality upfront so we will always be able to score."
Keller said their defensive problems relate to a lack of pressure from their midfielders.
"In a game one should always look at both sides of the coin," the 43-year-old said.
"Sure, you need to score goals to win a game but we've definitely conceded too many goals; that's why it's important that we play as focused as we did on Saturday.
"We all need to work to put pressure on the holder of the ball to get it back.
"But it'll also be very important to score goals.
"I've always said that if we are tight at the back, and work hard in the back we'll have enough quality upfront to score a goal. That's what we want to do tomorrow."
Keller wants his side to go in for the kill against an inconsistent Hoffenheim, as they eye a quarter-final spot in the German cup competition.
"Well, Hoffenheim is also a team where things go up and down," he said.
"They have some very good games but then also games that aren't so good, where they concede a lot of goals.
"In the last few weeks they didn't get as many points as they wanted so it's like a surprise package with them.
"They have a lot of quality in attack but they also show defensive frailties and that's what we need to take advantage of."
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