Dirk Nowitzki definitely plays Center as a secondary position. Here is the justification for this case.
1) 82games.com lists Nowitzki as playing Center as a secondary position. As the line indicates he plays 9% of the Center minutes for Dallas.
2. According to ESPN's 2008 draft packet Nowitzki plays Center.
"Positions get weird in the NBA -- is Nowitzki center-eligible? He starts at forward and plays on the perimeter but occasionally lines up at center."
3. Look at the Mavericks other options at Center. You have Erick Dampier, DeSagana Diop, and DJ Mbenga. According to 82games.com Dampier play 48% of the Center minutes, Diop played 37% and Mbenga played 1%. that leave 14% that isn't covered. At least 9% of which is covered by Dirk.
4. Other sites that list him as a Center
2. hoops.ws- The site we used to use for the league still has him as a C/F
"Dirk Nowitzki FC DAL 78 35.6 1916 24.6 692 8.9 263 3.4 52 0.7 62 0.8 167 44.2 1.24"