Bremen suffered a 2-0 defeat to new Bundesliga leaders Mainz on Saturday on the back of last week's 2-all draw with Tottenham Hotspur in the Champions League and have just four league points from four games.
"I do not remember a game where we were so bad," admitted Schaaf, who has been in charge of Bremen since 1999, after the Mainz defeat. "We have to do better."
Bremen captain Torsten Frings said his side must fight for the three points at Hannover to get their season back on track ahead of their next Champions League game against holders Inter Milan on September 29 in Italy.
"It's probably our worst game since I started wearing the jersey," admitted Frings, who made his debut for Bremen in 1997. "It makes life easier for our opponents if we defended badly, we didn't
fight in the individual duels."
Bremen are enduring their worst start to the league in nine years and have already been beaten twice this season by Hoffenheim and now Mainz.
Bremen hope to have centre-back Per Mertesacker back from injury on Tuesday and he is expected to play with a face mask after fracturing his eye socket playing for Germany in a Euro 2012 qualifier.
With four straight wins under their belt, new leaders Mainz are home to Cologne Tuesday looking to cement their advantage at the top of the table while teenage striker Andre Schürrle is looking for his fourth goal in four games.
Defending champions Bayern Munich need a convincing win at Hoffenheim on Tuesday after Saturday's goalless draw with Cologne at the Allianz Arena.
"We didn't take our chances again, and this time, the final pass wasn't always accurate enough," said Bayern coach van Gaal. "We did everything we could to get into dangerous positions in the box, but that's not enough. We've also got to score."
Second-placed Hoffenheim lost their 100 percent record in the Bundesliga on Saturday to Kaiserslautern and will be looking to take Bayern's scalp while keeping the pressure on leader Mainz.
On Wednesday, Schalke are at fourth-placed Freiburg and are desperate to avoid their fifth straight defeat after falling to a 3-1 home defeat to arch-rivals Dortmund on Sunday.