The Gelsenkirchen outfit sit third in Group E, one point behind their Swiss opponents.
Schalke therefore know only victory at the Veltins Arena will ensure progress to the round of 16, with Premier League outfit Chelsea having already claimed one of the qualification slots.
Former Basel midfielder Granit Xhaka has warned of the threat posed by the likes of Mohamed Salah in the Swiss champions' attack, and Keller knows his charges have to be wary of being hit on the counter in Wednesday's showdown.
"The players are ready, but we have to get the balance between defence and attack right," Keller said.
"Despite the fact we have to win the game it's important we don't get caught on the break. That would be the wrong plan. We have 90 minutes to score one more goal than our opponents.
"Even if we do go behind, nothing is lost. We have enough quality in the team to beat Basel a second time."
Schalke can take heart from having beaten Basel 1-0 in October's reverse fixture.
"My coaching team and I have gone through a video analysis of Basel's strengths and weaknesses with the players, though that's nothing out of the ordinary. We do it before every game," added Keller.
"We saw in the reverse fixture how Basel can be beaten. Tactically we played very well that night. We now need a similar performance at the Veltins Arena."