Unbeaten Leverkusen are three points clear at the top of the table, but second-placed Bremen need to beat Schalke, who are third, to keep the pressure on the league leaders.
Peru's Pizarro, who has scored six league goals in just eight games, has been out for four weeks with a foot injury and is expected to partner Germany's rising star Marko Marin up front in the clash at Bremen's Weserstadion.
"Up until now we have made a good start," said Schaaf, whose side has been beaten just once in the league after 15 matches. "Now we must show that we can win the big games and give a clear signal we are in the comptition."
Bremen will also have Germany trio goalkeeper Tim Wiese, plus midfielders Mesut Özil and Aaron Hunt back in the side with Leverkusen playing bottom side Hertha Berlin in the capital on Friday night.
Bayern Munich will be looking to pick up at Bochum where they left off in Turin having booked their place in the last 16 of the Champions League with a 4-1 win over Juventus last Tuesday.
After a poor start to the season, the win looks to have secured the future of Bayern's Louis van Gaal's side who are now up to fourth in the table.
Bochum are 15th in the league, but Germany forward Mario Gomez - who scored a stunning goal against Juventus - says the league game is crucial.
"We have another important match on Saturday," said the Bayern striker. "We have to follow up the result in Turin, and we're determined to do so."
Stuttgart might be 16th in the Bundesliga table and in the dreaded relegation zone, but having reached the Champions League's last 16, new coach Christian Gross will be hoping they can build on their European success.
Gross took over from Markus Babbel as coach last Sunday and enjoyed immediate success when his side beat Romanian champions Unirea Urziceni 3-1 on Wednesday.
With just two wins to their credit from 15 games, Stuttgart on Sunday need to beat Mainz, who are ninth, to get out of the bottom three and boost some low morale.
Ex-Germany goalkeeper Jens Lehmann landed himself in hot water in the wake of Babbel's dismissal by claiming his Stuttgart bosses bowed to pressure from angry fans.
Team manager Horst Heldt has said there will be consequences for Lehmann.
Having been dumped out of the Champions League on Tuesday after Manchester United beat them 3-1 at home, defending German champions Wolfsburg have the chance to get their league campaign back on track at home to Dortmund on Sunday