I wouldn't put much stock in Detroit losing to Dallas during the second week of the season. Detroit was 8-0 at the point and was due for a let down. Dallas played that game like it was their Super Bowl. In their second meeting Detroit handily beat Mavericks and the score does not show how dominant the Pistons were in that game.
The point is mute because Dallas has about a 10% chance of beating San Antonio this series. They don't have enough toughness to get past the Spurs. As Jerry Stackhouse put it, "I've always thought the Mavericks were a soft team." Hey, can't argue with a guy who plays for the Mavs.