Bremen will be fighting for their lives with the gap to the relegation zone now down to seven points with as many games of the season remaining, yet Keller says they have been blowing hot and cold all season and are therefore an unpredictable prospect.
"They are incredibly difficult to fathom," he said. "Sometimes they play fantastic, world class football but here and there they also throw in a not so good performance.
"They are on a bit of a downer at the moment, but they will want to get out of it in front of their own fans, which is why I expect a very difficult game against highly-engaged opponents.
"What must not happen is that we don't give it our best in Bremen after winning our last game at home."
That 3-0 win over Hoffenheim lifted the Royal Blues to fourth in the table, above Eintracht Frankfurt on goals scored, and Keller says his side "need to collect more points" to achieve their objective for the season, which is a place in the Champions League.
Bremen can no longer harbour such dreams as they are set to miss out on a place in Europe for the third year in a row, but their coach Thomas Schaaf insists that was never their objective for the season.
"It's important that things are viewed realistically and that people view Werder Bremen's potential correctly," he told Radio Bremen in response to recent criticism from the fans.
"We never made any promises about wanting to qualify for the Champions League or the Europa League.
"We want to finish in a challenging position and we are lagging just a tick behind, but not too much."
Both teams have injury worries with Clemens Fritz, Lukas Schmitz and Zlatko Junuzovic unavailable for the home team while Schalke's long list of absentees includes Klaas-Jan Huntelaar, Kyriakos Papadopoulos, Jermaine Jones, Chinedu Obasi and Ibrahim Afellay.
"It's true that we are down to the bare bones at the moment in terms of players, but we never moaned in the past and we have enough quality to win in Bremen anyway," said Keller on his club's website.