Probable starting eleven:

Dortmund:
Weidenfeller – Piszczek, Subotic, Hummels, Schmelzer – Sahin, Bender – Goetze, Kagawa, Grosskreutz – Barrios

Hamburg:
Drobny – Demel, Westermann, Mathijsen, Ze Roberto – Kacar, Jarolim, Trochowski – Pitroipa, Petric, Guerrero

I see three reasons for Under 2.5 goals on this match. Let's go one by one:

1. Both teams are incomplete for this match. Some of the missing players are very important, like Hamburg best striker Ruud van Nistelrooy. But there are much more. Hamburg coach Armin Veh will be without Frank Rost, Marcell Jansen, Collin Benjamin, Dennis Aogo, Dennis Diekmeier, Eljero Elia, Tunay Torun, Miroslav Stepanek and Romeo Castelen. I must say the list is very long. On other side Dortmund coach Klopp will be without injured Sebastian Kehl, Patrick Owomoyela, and Florian Kringe. Two more players are just back from injuries and probably won't play - Tamas Hajnal and Mohamed Zidan.

2. The tradition. Just one match in the last six ended with Over. Here are the results:

Dortmund - Hamburger 1:0 23.01.2010
Dortmund - Hamburger 2:0 25.04.2009
Dortmund - Hamburger 0:3 25.09.2007
Dortmund - Hamburger 1:0 16.09.2006
Dortmund - Hamburger 1:1 23.10.2005
Dortmund - Hamburger 0:2 23.10.2004

3. And finally, I don't expect Hamburg to score at Signal-Iduna Park in Dortmund. Borussia Dortmund have the best defense in Bundesliga, specially at home where they received only one goal so far. Hamburg is without van Nistelrooy and in such situation I believe they won't score. So, under 2.5 goals is the best option for this game.