I think most knowledgeable fans realize that the Mavericks are the favorites to win this series. The Spurs are the 7th seed for a reason and its not because they wanted a new challenge.
I think the Spurs CAN win this series but they need for a lot of things to fall into place:
1. The Big 3 need to play like the Big 3. If any of these guys does not play up to their potential then the odds of winning will drop dramatically. Manu is going to have to do the heaviest lifting but unless Duncan and Parker also contribute at all star levels then the Mavs are probably going to dominate this series.
2. We're going to have to get solid role playing contributions in every game. This doesn't mean that Hill, Jefferson and others have to play amazingly ever single game but the fact of the matter is that the Spurs will need SOMEONE to step up each and every game to provide a fourth option and reliable bench scoring. It could be Hill one game and Jefferson another but they will need someone to step up and help the Big 3.
3. Jefferson will have to play good to great defense on Dirk. Hill is too small, Hairston isn't likely to get much of a shot and is probably too light anyway but Jefferson provides the best chance we have at limiting Dirk from a superstar performance. We don't need him to channel Bowen but we do need him to provide good defense and prevent Dirk from having amazing games. Good games we can live with, Amazing games not so much.
They may seem obvious but the lack of consistency on these issues is a huge reason the Spurs are the 7th seed and not the 2nd seed.