The Peruvian, who has scored 11 goals in 13 league games this season, missed training this week with coach Thomas Schaaf saying he hoped Pizarro could be fit for Sunday's game.
Werder are in fifth place on 23 points, five off leaders Bayern Munich.
Hamburg will go to Hannover on Saturday without striker Mladen Petric, who has not yet fully recovered from an injury.
"He has been feeling something this week. It is not a new injury but we have to be careful so he will be out for this match," said coach Thorsten Fink.
Hamburg lifted themselves off the relegation zone for the first time in months with a 2-0 win over Hoffenheim last week and are unbeaten in their last five league games.
Dutch defender Roel Brouwers has extend his contract at third-placed Borussia Moenchengladbach by two years to 2014, the club said.
"Roel is a crucial piece in our defensive operation, an exemplary professional you can always rely on," said club sports director Max Eberl ahead of Friday's trip to Cologne.
Hitzlsperger, who joined Wolfsburg in the summer, said he did not know when he would be fully fit for a comeback.
Wolfsburg, the 2009 champions, have been struggling this season and are in 12th place. They travel to bottom-placed Augsburg on Saturday.